NIST Special Publication 1019-5

Fire Dynamics Simulator (Version 5)User’s Guide

Kevin McGrattanRandall McDermott

Simo HostikkaJason Floyd

In cooperation with:VTT Technical Research Centre of Finland

NIST Special Publication 1019-5

Fire Dynamics Simulator (Version 5)User’s Guide

Kevin McGrattanRandall McDermott

NIST Building and Fire Research LaboratoryGaithersburg, Maryland, USA

Simo HostikkaVTT Technical Research Centre of Finland

Espoo, Finland

Jason FloydHughes Associates, Inc.

Baltimore, Maryland, USA

April 14, 2010FDS Version 5.5

SV NRepository Revision : 6055

UN

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DSTATES OF AM

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PARTMENT OF COMMERC

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U.S. Department of CommerceGary Locke, Secretary

National Institute of Standards and TechnologyPatrick Gallagher, Director

Certain commercial entities, equipment, or materials may be identified in thisdocument in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by theNational Institute of Standards and Technology, nor is it intended to imply that theentities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 1019-5Natl. Inst. Stand. Technol. Spec. Publ. 1019-5, 208 pages (October 2007)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICEWASHINGTON: 2007

For sale by the Superintendent of Documents, U.S. Government Printing OfficeInternet: bookstore.gpo.gov – Phone: (202) 512-1800 – Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001

Preface

This Guide describes how to use the Fire Dynamics Simulator (FDS) version 5. Because new features areadded periodically, check the current version number on the inside front jacket of this manual.

Note that this Guide does not provide the background theory for FDS. A four volume set of companiondocuments, referred to collectively as the FDS Technical Reference Guide [1], contains details about thegoverning equations and numerical methods, model verification, experimental validation, and configurationmanagement. The FDS User’s Guide contains limited information on how to operate Smokeview, the com-panion visualization program for FDS. Its full capability is described in the Smokeview User’s Guide [2].

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Disclaimer

The US Department of Commerce makes no warranty, expressed or implied, to users of the Fire DynamicsSimulator (FDS), and accepts no responsibility for its use. Users of FDS assume sole responsibility underFederal law for determining the appropriateness of its use in any particular application; for any conclusionsdrawn from the results of its use; and for any actions taken or not taken as a result of analyses performedusing these tools.

Users are warned that FDS is intended for use only by those competent in the fields of fluid dynamics,thermodynamics, combustion, and heat transfer, and is intended only to supplement the informed judgmentof the qualified user. The software package is a computer model that may or may not have predictivecapability when applied to a specific set of factual circumstances. Lack of accurate predictions by the modelcould lead to erroneous conclusions with regard to fire safety. All results should be evaluated by an informeduser.

Throughout this document, the mention of computer hardware or commercial software does not con-stitute endorsement by NIST, nor does it indicate that the products are necessarily those best suited for theintended purpose.

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About the Authors

Kevin McGrattan is a mathematician in the Building and Fire Research Laboratory of NIST. He receiveda bachelors of science degree from the School of Engineering and Applied Science of Columbia Uni-versity in 1987 and a doctorate at the Courant Institute of New York University in 1991. He joinedthe NIST staff in 1992 and has since worked on the development of fire models, most notably the FireDynamics Simulator.

Randall McDermott joined the research staff of the Building and Fire Research Lab in 2008. He receiveda B.S. degree from the University of Tulsa in Chemical Engineering in 1994 and a doctorate at theUniversity of Utah in 2005. His research interests include subgrid-scale models and numerical methodsfor large-eddy simulation, adaptive mesh refinement, immersed boundary methods, and Lagrangianparticle methods.

Simo Hostikka is a Senior Research Scientist at VTT Technical Research Centre of Finland. He received amaster of science (technology) degree in 1997 and a doctorate in 2008 from the Department of Engineer-ing Physics and Mathematics of the Helsinki University of Technology. He is the principal developer ofthe radiation and solid phase sub-models within FDS.

Jason Floyd is a Senior Engineer at Hughes Associates, Inc., in Baltimore, Maryland. He received a bach-elors of science degree and a doctorate from the Nuclear Engineering Program of the University ofMaryland. After graduating, he won a National Research Council Post-Doctoral Fellowship at theBuilding and Fire Research Laboratory of NIST, where he developed the combustion algorithm withinFDS. He is currently funded by the Fire Research Grants Program. He is the principal developer of themulti-parameter mixture fraction combustion model and control logic within FDS.

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Acknowledgments

The Fire Dynamics Simulator, in various forms, has been under development for almost 25 years. However,the publicly released software has only existed since 2000. Since its first release, continued improvementshave been made to the software based largely on feedback from its users. Included here are some who madeimportant contributions.

At NIST, thanks to Dan Madrzykowski, Doug Walton, Bob Vettori, Dave Stroup, Steve Kerber andNelson Bryner, who have used FDS and Smokeview as part of several investigations of fire fighter line ofduty deaths. As part of these studies, they have provided valuable information on the model’s usability andaccuracy when compared to large scale measurements made during fire reconstructions.

Bryan Klein of NIST assisted in adding cross-referencing functionality to this document, making iteasier to view electronically.

The US Nuclear Regulatory Commission has provided financial support for the maintenance and de-velopment of FDS, along with valuable insights into how fire models are used as part of probabilistic riskassessments of nuclear facilities. Special thanks to Mark Salley and Jason Dreisbach of NRC, and FranciscoJoglar of SAIC.

The Society of Fire Protection Engineers (SFPE) sponsors a training course on the use of FDS andSmokeview. Chris Wood of ArupFire, Dave Sheppard of the US Bureau of Alcohol, Tobacco and Firearms(ATF), and Doug Carpenter of Combustion Science and Engineering developed the materials for the course,along with Morgan Hurley of the SFPE.

Prof. David McGill of Seneca College, Ontario, Canada has conducted a remote-learning course on theuse of FDS, and he has also maintained a web site that has provided valuable suggestions from users.

Prof. Ian Thomas of Victoria University has also presented short courses on the use of FDS in Australia.His students have also performed some validation work on compartment fires.

Prof. Charles Fleischmann and his students at the University of Canterbury, New Zealand, have providedvaluable assistance in improving the documentation and usability of the model.

James White Jr. of the Western Fire Center has provided valuable feedback on how to improve thefunctionality of the model in the area of forensic science.

Paul Hart of Swiss Re, GAP Services, and Pravinray Gandhi of Underwriters Laboratories provideduseful suggestions about water droplet transport on solid objects.

Dr. Chris Lautenberger of University of California, Berkeley, has helped in development and improvingthe documentation of the pyrolysis models.

Finally, on the following pages is a list of individuals and organizations who have volunteered their timeand effort to “beta test” FDS and Smokeview prior to its official release. Their contribution is invaluablebecause there is simply no other way to test all of the various features of the model.

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FDS 5 Beta TestersNick Agnew Maunsell, AustraliaCamille Azzi Universities of Glasgow and Strathclyde, ScotlandMatthew Bilson Maunsell, AustraliaGeorge Braga Federal District Fire Department, BrazilKeith Calder Senez Reed Calder Engineering, CanadaSteven Chi Heng Lam Hoare Lea Fire Engineering, UKDoo Chan Choi Rolf Jensen & Associates, Inc., USAMarco Cigolini Italferr spa, ItalyJohn Cutonilli Hughes Associates, Inc., USASylvain Desanghere CTICM (Centre Technique Industriel de la Construction Métallique), FranceMontu L. Das Gage-Babcock & Associates, USA and CanadaFranck Didieux Laboratoire National de Métrologie et d’Essais (LNE), FranceJohannes Dimyadi AstraVision-Solutions, New ZealandBill Ferrante Roosevelt Fire District, USAPaul Fuss NIST, USARalf Galster Ing. Büro für Brandschutz Riesener, GermanyAndreas Gerndt University of Louisiana, USAEmanuele Gissi Corpo Nazionale dei Vigili del Fuoco, Comando Prov. di Genova, ItalySimon J. Ham Fire Safety Engineering Consultants Ltd., UKPaul Hart Swiss Re, GAP Services, USAHsiao, Li Kai (Gary) Fire Bureau, Taipei, TaiwanHu Zhi-Xin University of Maryland, USAIlya N. Karkin SITIS Ltd., RussiaSusanne Kilian hhpberlin, Fire Safety Engineers, GermanySung Chan Kim School of Mechanical Engineering, Chung Ang University, KoreaPierre-Louis Lamballais Flashover-Backdraft, FranceA. Leonardi StIL (Studio di Ingegneria Leonardi), ItalyDavy Leroy Arup Fire, UKYunlong Liu Parsons Brinckerhof, AustraliaTimothy Liu Locke Carey Fire Consultants, UKDave McGill Seneca College, Ontario, CanadaKen Miller Las Vegas Fire & Rescue, USAStephan B. Miller Mr. 3D Computer Graphics and the University of Houston Downtown, USAPete Muir Safe Consulting, UKStephen Olenick Combustion Science & Engineering, Inc., USAKristopher Overholt University of Houston Downtown, USAPENG Wei State Key Labortory of Fire Science, ChinaAndrew Purchase Maunsell, AustraliaChristian Rogsch University of Wuppertal, GermanyMichael Roth RWDI, CanadaAhmed Salem Alexandria University, EgyptRobert Schmidt Combustion Science & Engineering, Inc., USAJoe Skaggs CASE Forensics, USAPiotr Smardz Ahearne Fire Engineering Consultants, Ireland

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Jamie Stern-Gottfried Arup Fire, UKBoris Stock BFT Cognos GmbH, GermanyBlair Stratton Beca, New ZealandCsaba Szilagyi Szolnok Fire Department, HungaryMahmood Tabaddor Underwriters Laboratories, USACharlie Thornton Thunderhead Engineering, USASebastian Ukleja University of Ulster, Northern IrelandGiacomo Villi Dipartimento Fisica Tecnica (DfT) UNIPd, ItalyAndreas Vischer RWTH Aachen University, GermanyKarl Wallasch Hoare Lea Fire Engineering, UKKaoru Wakatsuki National Research Institute of Fire and Disaster, JapanJoachim Wollstädt Ing. Büro für Brandschutz Riesener, GermanyYang Shan-You State Key Laboratory of Fire Science, ChinaMatthias Zähringer Ing. Büro für Brandschutz Riesener, GermanyRobin Zevotek C&S Engineers, Life Safety Services, Syracuse, New York, USA

GIDAI, University of Cantabria, Spain

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Contents

Preface i

Disclaimer iii

About the Authors v

Acknowledgments vii

I The Basics of FDS 1

1 Introduction 31.1 Features of FDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 What’s New in FDS 5? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Getting Started 72.1 How to Acquire FDS and Smokeview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Computer Hardware Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3 Computer Operating System (OS) and Software Requirements . . . . . . . . . . . . . . . 8

3 Running FDS 93.1 Starting an FDS Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Starting an FDS Calculation (Single Processor Version) . . . . . . . . . . . . . . . 93.1.2 Starting an FDS Calculation (Multiple Processor Version) . . . . . . . . . . . . . . 10

3.2 Monitoring Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 User Support 154.1 The Version Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Common Error Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3 Support Requests and Bug Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.4 Known Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

II Writing an FDS Input File 21

5 The Basic Structure of an Input File 235.1 Naming the Job . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.2 Namelist Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.3 Input File Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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6 Setting the Bounds of Time and Space 276.1 Naming the Job: The HEAD Namelist Group (Table 15.6) . . . . . . . . . . . . . . . . . . 276.2 Simulation Time: The TIME Namelist Group (Table 15.26) . . . . . . . . . . . . . . . . . 27

6.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.2.2 Special Topic: Controlling the Time Step . . . . . . . . . . . . . . . . . . . . . . . 286.2.3 Special Topic: Steady-State Applications . . . . . . . . . . . . . . . . . . . . . . . 28

6.3 Computational Meshes: The MESH Namelist Group (Table 15.11) . . . . . . . . . . . . . 296.3.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.3.2 Two-Dimensional and Axially-Symmetric Calculations . . . . . . . . . . . . . . . . 296.3.3 Multiple Meshes and Parallel Processing . . . . . . . . . . . . . . . . . . . . . . . 306.3.4 Mesh Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316.3.5 Mesh Stretching: The TRNX, TRNY and/or TRNZ Namelist Groups (Table 15.27) . . 336.3.6 Choosing Optimum Mesh Dimensions . . . . . . . . . . . . . . . . . . . . . . . . 35

6.4 Miscellaneous Parameters: The MISC Namelist Group (Table 15.12) . . . . . . . . . . . . 366.4.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.4.2 Special Topic: Stopping and Restarting Calculations . . . . . . . . . . . . . . . . . 376.4.3 Special Topic: Defying Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.4.4 Special Topic: The Baroclinic Vorticity . . . . . . . . . . . . . . . . . . . . . . . . 386.4.5 Special Topic: Stack Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.4.6 Special Topic: Large Eddy Simulation Parameters . . . . . . . . . . . . . . . . . . 406.4.7 Special Topic: Numerical Stability Parameters . . . . . . . . . . . . . . . . . . . . 40

6.5 Special Topic: Unusual Initial Conditions: The INIT Namelist Group (Table 15.8) . . . . 426.6 Special Topic: Improving the Pressure Solver: The PRES Namelist Group (Table 15.16) . . 426.7 Special Topic: Setting Limits: The CLIP Namelist Group (Table 15.2) . . . . . . . . . . . 43

7 Building the Model 457.1 Bounding Surfaces: The SURF Namelist Group (Table 15.24) . . . . . . . . . . . . . . . . 457.2 Creating Obstructions: The OBST Namelist Group (Table 15.14) . . . . . . . . . . . . . . 45

7.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457.2.2 Repeated Obstructions: The MULT Namelist Group (Table 15.13) . . . . . . . . . . 477.2.3 Non-rectangular Geometry and Sloped Ceilings . . . . . . . . . . . . . . . . . . . . 48

7.3 Creating Voids: The HOLE Namelist Group (Table 15.7) . . . . . . . . . . . . . . . . . . . 507.4 Applying Surface Properties: The VENT Namelist Group (Table 15.28) . . . . . . . . . . . 51

7.4.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517.4.2 Special VENTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527.4.3 Controlling VENTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.4.4 Trouble-Shooting VENTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

7.5 Coloring Obstructions, Vents, Surfaces and Meshes . . . . . . . . . . . . . . . . . . . . . 547.5.1 Texture Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8 Fire and Thermal Boundary Conditions 578.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578.2 Surface Temperature and Heat Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

8.2.1 Specified Solid Surface Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 588.2.2 Special Topic: Convective Heat Transfer Options . . . . . . . . . . . . . . . . . . . 588.2.3 Special Topic: Adiabatic Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

8.3 Heat Conduction in Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 608.3.1 Structure of Solid Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

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8.3.2 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618.3.3 Back Side Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628.3.4 Initial and Back Side Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . 628.3.5 Walls with Different Materials Front and Back . . . . . . . . . . . . . . . . . . . . 628.3.6 Special Topic: Non-Planar Walls and Targets . . . . . . . . . . . . . . . . . . . . . 638.3.7 Special Topic: Solid Phase Numerical Gridding Issues . . . . . . . . . . . . . . . . 63

8.4 Pyrolysis Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658.4.1 A Gas Burner with a Specified Heat Release Rate . . . . . . . . . . . . . . . . . . . 658.4.2 Special Topic: A Radially-Spreading Fire . . . . . . . . . . . . . . . . . . . . . . . 658.4.3 Solid Fuels that Burn at a Specified Rate . . . . . . . . . . . . . . . . . . . . . . . 668.4.4 Solid Fuels that do NOT Burn at a Specified Rate . . . . . . . . . . . . . . . . . . . 678.4.5 Liquid Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738.4.6 Fuel Burnout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

8.5 Testing Your Pyrolysis Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

9 Ventilation 839.1 Simple Vents, Fans and Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

9.1.1 Supply and Exhaust Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839.1.2 Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849.1.3 Total Mass Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849.1.4 Louvered Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849.1.5 Jet Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

9.2 Species and Species Mass Flux Boundary Conditions . . . . . . . . . . . . . . . . . . . . 859.3 Special Topic: Pressure Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . 859.4 Special Topic: Fires and Flows in the Outdoors . . . . . . . . . . . . . . . . . . . . . . . 869.5 Tangential Velocity Boundary Conditions at Solid Surfaces . . . . . . . . . . . . . . . . . 879.6 Pressure-Related Effects: The ZONE Namelist Group (Table 15.28) . . . . . . . . . . . . . 88

9.6.1 Specifying Pressure Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 889.6.2 Leaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 899.6.3 Fan Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

10 User-Specified Functions 9510.1 Time-Dependent Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9510.2 Temperature-Dependent Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9610.3 Tabular Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

11 Combustion and Radiation 9911.1 Mixture Fraction Combustion: The REAC Namelist Group . . . . . . . . . . . . . . . . . 99

11.1.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9911.1.2 Special Topic: Heat of Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . 10211.1.3 Special Topic: Flame Extinction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10211.1.4 Special Topic: CO Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10311.1.5 Special Topic: Turbulent Combustion . . . . . . . . . . . . . . . . . . . . . . . . . 103

11.2 Extra Gas Species: The SPEC Namelist Group . . . . . . . . . . . . . . . . . . . . . . . . 10511.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10511.2.2 Special Topic: Gas Species Properties . . . . . . . . . . . . . . . . . . . . . . . . . 10611.2.3 Special Topic: Yields of Gaseous Species (NU_GAS) . . . . . . . . . . . . . . . . 10711.2.4 Special Topic: Finite-Rate Combustion . . . . . . . . . . . . . . . . . . . . . . . . 108

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11.3 Radiation Transport: The RADI Namelist Group . . . . . . . . . . . . . . . . . . . . . . . 110

12 Particles and Droplets 11112.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11112.2 Particle and Droplet Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

12.2.1 Particles Introduced at a Solid Surface . . . . . . . . . . . . . . . . . . . . . . . . . 11212.2.2 Droplets Introduced at a Sprinkler or Nozzle . . . . . . . . . . . . . . . . . . . . . 11212.2.3 Particles or Droplets Introduced within a Volume . . . . . . . . . . . . . . . . . . . 11312.2.4 Controlling the Number of Particles and Droplets . . . . . . . . . . . . . . . . . . . 113

12.3 Particle and Droplet Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11312.3.1 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11412.3.2 Size Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11412.3.3 Drag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11412.3.4 Velocity on Solid Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11512.3.5 Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

12.4 Special Types of Particles and Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11612.4.1 Massless Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11612.4.2 Static Particles or Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11612.4.3 Water Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11612.4.4 Fuel Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11612.4.5 Solid Particles that do not Evaporate . . . . . . . . . . . . . . . . . . . . . . . . . . 117

12.5 Special Topic: Suppression by Water (Mixture Fraction Model Only) . . . . . . . . . . . . 118

13 Devices and Control Logic 12113.1 Device Location and Orientation: The DEVC Namelist Group (Table 15.4) . . . . . . . . . 12113.2 Device Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12213.3 Special Devices and their Properties: The PROP Namelist Group (Table 15.18) . . . . . . 123

13.3.1 Sprinklers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12313.3.2 Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12513.3.3 Heat Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12613.3.4 Smoke Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12713.3.5 Beam Detection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12813.3.6 Aspiration Detection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12913.3.7 Electrical Cable Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

13.4 Basic Control Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13313.4.1 Creating and Removing Obstructions . . . . . . . . . . . . . . . . . . . . . . . . . 13313.4.2 Activating and Deactivating Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

13.5 Advanced Control Functions: The CTRL Namelist Group . . . . . . . . . . . . . . . . . . 13513.5.1 Control Functions: ANY, ALL, ONLY, and AT_LEAST . . . . . . . . . . . . . . . . 13513.5.2 Control Function: TIME_DELAY . . . . . . . . . . . . . . . . . . . . . . . . . . . 13613.5.3 Control Function: DEADBAND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13613.5.4 Control Function: RESTART and KILL . . . . . . . . . . . . . . . . . . . . . . . 13713.5.5 Control Function: CUSTOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13713.5.6 Combining Control Functions: A Pre-Action Sprinkler System . . . . . . . . . . . . 13813.5.7 Combining Control Functions: A Dry Pipe Sprinkler System . . . . . . . . . . . . . 13813.5.8 Example Case: activate_vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

13.6 Controlling a RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13913.7 Visualizing FDS Devices Using Smokeview Objects . . . . . . . . . . . . . . . . . . . . . 140

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13.7.1 Static Smokeview Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14013.7.2 Dynamic Smokeview Objects - Customized Using &PROP Parameters . . . . . . . 14213.7.3 Dynamic Smokeview Objects - Customized Using &PROP Parameters and Particle

File Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

14 Output Data 14714.1 Output Control Parameters: The DUMP Namelist Group . . . . . . . . . . . . . . . . . . . 14714.2 Output File Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

14.2.1 Device Output: The DEVC Namelist Group . . . . . . . . . . . . . . . . . . . . . . 14914.2.2 Quantities within Solids: The PROF Namelist Group . . . . . . . . . . . . . . . . . 15014.2.3 Animated Planar Slices: The SLCF Namelist Group . . . . . . . . . . . . . . . . . 15014.2.4 Animated Boundary Quantities: The BNDF Namelist Group . . . . . . . . . . . . . 15114.2.5 Animated Isosurfaces: The ISOF Namelist Group . . . . . . . . . . . . . . . . . . 15114.2.6 Plot3D Static Data Dumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15114.2.7 SMOKE3D: Realistic Smoke and Fire . . . . . . . . . . . . . . . . . . . . . . . . . 152

14.3 Special Output Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15314.3.1 Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15314.3.2 Visibility and Obscuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15414.3.3 Layer Height and the Average Upper and Lower Layer Temperatures . . . . . . . . 15514.3.4 The True Gas Temperature vs. the Measured Gas Temperature . . . . . . . . . . . . 15514.3.5 Heat Fluxes and Thermal Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 15614.3.6 Droplet Output Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15614.3.7 Interfacing with Structural Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 15814.3.8 Useful Solid Phase Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15914.3.9 Fractional Effective Dose (FED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15914.3.10 Spatially-Integrated Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16014.3.11 Temporally-Integrated Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16214.3.12 Wind and the Pressure Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . 16214.3.13 Dry Volume and Mass Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16314.3.14 Gas Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

14.4 Extracting Numbers from the Output Data Files . . . . . . . . . . . . . . . . . . . . . . . 16314.5 Summary of Frequently-Used Output Quantities . . . . . . . . . . . . . . . . . . . . . . . 16514.6 Summary of Infrequently-Used Output Quantities . . . . . . . . . . . . . . . . . . . . . . 169

15 Alphabetical List of Input Parameters 17115.1 BNDF (Boundary File Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.2 CLIP (MIN/MAX Clipping Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.3 CTRL (Control Function Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.4 DEVC (Device Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17315.5 DUMP (Output Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17315.6 HEAD (Header Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17415.7 HOLE (Obstruction Cutout Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17415.8 INIT (Initial Conditions) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.9 ISOF (Isosurface Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.10 MATL (Material Properties) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.11 MESH (Mesh Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17615.12 MISC (Miscellaneous Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17715.13 MULT (Multiplier Function Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

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15.14 OBST (Obstruction Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17815.15 PART (Lagrangian Particles/Droplets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17915.16 PRES (Pressure Solver Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18015.17 PROF (Wall Profile Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18015.18 PROP (Device Properties) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18015.19 RADI (Radiation Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18115.20 RAMP (Ramp Function Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18215.21 REAC (Reaction Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18215.22 SLCF (Slice File Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18315.23 SPEC (Species Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18415.24 SURF (Surface Properties) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18415.25 TABL (Table Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.26 TIME (Time Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.27 TRNX, TRNY, TRNZ (MESH Transformations) . . . . . . . . . . . . . . . . . . . . . . 18615.28 VENT (Vent Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.29 ZONE (Pressure Zone Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

16 Conversion of Old Input Files to FDS 5 18916.1 Numerical Domain Parameters: GRID and PDIM . . . . . . . . . . . . . . . . . . . . . . 18916.2 Obstructions, Vents, and Holes: OBST, VENT, and HOLE . . . . . . . . . . . . . . . . . . 18916.3 Surface Parameters: SURF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18916.4 Reaction Parameters: REAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19016.5 Device Parameters: SPRK, SMOD, HEAT, THCP . . . . . . . . . . . . . . . . . . . . . . . 191

III FDS and Smokeview Development Tools 193

17 The FDS/Smokeview Repository 195

18 Compiling FDS 19718.1 FDS Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

19 Output File Formats 19919.1 Diagnostic Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19919.2 Heat Release Rate and Related Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . 20019.3 Device Output Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20019.4 Control Output Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20119.5 Gas Mass Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20119.6 Mixture Fraction State Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20119.7 Slice Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20119.8 Plot3D Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20219.9 Boundary Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20319.10 Particle Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20319.11 Profile Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20419.12 3-D Smoke File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20419.13 Isosurface File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Bibliography 207

xv

List of Figures

6.1 An example of a multiple-mesh geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2 Rules governing the alignment of meshes. . . . . . . . . . . . . . . . . . . . . . . . . . . 326.3 Piecewise-Linear Mesh Transformation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.4 Polynomial Mesh Transformation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.5 Axi-symmetric helium plume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.6 Simple example of flow through a duct. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.1 An example of the multiplier function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487.2 Simple example of SAWTOOTH=.FALSE. . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8.1 Simple demonstration of pyrolysis model. . . . . . . . . . . . . . . . . . . . . . . . . . . 698.2 A more complicated demonstration of the pyrolysis model. . . . . . . . . . . . . . . . . . 718.3 Input parameters for sample case pyrolysis_2. . . . . . . . . . . . . . . . . . . . . . . . . 728.4 Input parameters for sample case ethanol_pan. . . . . . . . . . . . . . . . . . . . . . . . 738.5 Output of box_burn_away and rbox_burn_away2 test cases. . . . . . . . . . . . . . . . . 758.6 Output of thermoplastic test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788.7 Output of charring_solid test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 798.8 Output of room_fire test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

9.1 Example of positive pressure at a tunnel entrance. . . . . . . . . . . . . . . . . . . . . . . 869.2 Output of pressure_rise test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 909.3 Output of zone_break test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 919.4 Example of a fan curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 929.5 Output of the fan_test example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939.6 Pressure rise in sealed compartment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

11.1 Output of door_crack test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10311.2 Example of gas filling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

12.1 Example of water cascading over solid obstructions. . . . . . . . . . . . . . . . . . . . . . 11512.2 HRR of spray burner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

13.1 Output of the flow rate test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12713.2 Example of a beam detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12913.3 Output of aspiration_detector test case. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13113.4 Example of a vent controls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

xvi

List of Tables

4.1 FDS features with known issues or problems. . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1 Namelist Group Reference Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.1 Sample of Color Definitions (A complete list is included on the website) . . . . . . . . . . 55

11.1 Optional Gas Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

13.1 Suggested Values for Smoke Detector Model. . . . . . . . . . . . . . . . . . . . . . . . . 12813.2 Control function types for CTRL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13513.3 Single Frame Static Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14113.4 Dual Frame Static Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14113.4 Dual Frame Static Objects (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14213.5 Dynamic Objects - Customized using SMOKEVIEW_PARAMETERS on a &PROP line . 14313.5 Dynamic Objects (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14413.6 Dynamic Objects - Customized using SMOKEVIEW_PARAMETERS on a &PROP line

and Particle File Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14513.6 Dynamic Objects (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

14.1 Output quantities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16614.2 Output quantities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

15.1 Boundary File Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.2 MIN/MAX Clipping Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.3 Control Function Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17215.4 Device Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17315.5 Output Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17315.6 Header Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17415.7 Obstruction Cutout Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17415.8 Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.9 Isosurface Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.10 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17515.11 Mesh Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17615.12 Miscellaneous Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17715.13 Multiplier Function Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17815.14 Obstruction Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17815.15 Lagrangian Particles/Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17915.16 Pressure Solver Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18015.17 Wall Profile Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18015.18 Device Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

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15.19 Radiation Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18215.20 Ramp Function Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18215.21 Reaction Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18215.22 Slice File Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18315.23 Species Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18415.24 Surface Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18415.25 Table Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.26 Time Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.27 MESH Transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18615.28 Vent Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18715.29 Pressure Zone Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

18.1 Source Code Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

xviii

Part I

The Basics of FDS

1

Chapter 1

Introduction

The software described in this document, Fire Dynamics Simulator (FDS), is a computational fluid dynamics(CFD) model of fire-driven fluid flow. FDS solves numerically a form of the Navier-Stokes equationsappropriate for low-speed, thermally-driven flow with an emphasis on smoke and heat transport from fires.The formulation of the equations and the numerical algorithm are contained the FDS Technical ReferenceGuide [1].

Smokeview is a separate visualization program that is used to display the results of an FDS simulation.A detailed description of Smokeview is found in a separate user’s guide [2].

1.1 Features of FDS

The first version of FDS was publicly released in February 2000. To date, about half of the applications ofthe model have been for design of smoke handling systems and sprinkler/detector activation studies. Theother half consist of residential and industrial fire reconstructions. Throughout its development, FDS hasbeen aimed at solving practical fire problems in fire protection engineering, while at the same time providinga tool to study fundamental fire dynamics and combustion.

Hydrodynamic Model FDS solves numerically a form of the Navier-Stokes equations appropriate for low-speed, thermally-driven flow with an emphasis on smoke and heat transport from fires. The core algo-rithm is an explicit predictor-corrector scheme, second order accurate in space and time. Turbulence istreated by means of the Smagorinsky form of Large Eddy Simulation (LES). It is possible to perform aDirect Numerical Simulation (DNS) if the underlying numerical mesh is fine enough. LES is the defaultmode of operation.

Combustion Model For most applications, FDS uses a single step chemical reaction whose products aretracked via a two-parameter mixture fraction model. The mixture fraction is a conserved scalar quantitythat represents the mass fraction of one or more components of the gas at a given point in the flowfield. By default, two components of the mixture fraction are explicitly computed. The first is themass fraction of unburned fuel and the second is the mass fraction of burned fuel (i.e. the mass ofthe combustion products that originated as fuel). A two-step chemical reaction with a three parametermixture fraction decomposition can also be used with the first step being oxidation of fuel to carbonmonoxide and the second step the oxidation of carbon monoxide to carbon dioxide. The three mixturefraction components for the two step reaction are unburned fuel, mass of fuel that has completed thefirst reaction step, and the mass of fuel that has completed the second reaction step. The mass fractionsof all of the major reactants and products can be derived from the mixture fraction parameters by meansof “state relations.” Lastly, a multiple-step finite rate model is also available.

3

Radiation Transport Radiative heat transfer is included in the model via the solution of the radiation trans-port equation for a gray gas, and in some limited cases using a wide band model. The equation is solvedusing a technique similar to finite volume methods for convective transport, thus the name given to itis the Finite Volume Method (FVM). Using approximately 100 discrete angles, the finite volume solverrequires about 20 % of the total CPU time of a calculation, a modest cost given the complexity of radi-ation heat transfer. The absorption coefficients of the gas-soot mixtures are computed using RADCALnarrow-band model. Liquid droplets can absorb and scatter thermal radiation. This is important in casesinvolving mist sprinklers, but also plays a role in all sprinkler cases. The absorption and scatteringcoefficients are based on Mie theory.

Geometry FDS approximates the governing equations on a rectilinear mesh. Rectangular obstructions areforced to conform with the underlying mesh.

Multiple Meshes This is a term used to describe the use of more than one rectangular mesh in a calculation.It is possible to prescribe more than one rectangular mesh to handle cases where the computationaldomain is not easily embedded within a single mesh.

Parallel Processing It is possible to run an FDS calculation on more than one computer using the MessagePassing Interface (MPI). Details can be found in Section 3.1.2.

Boundary Conditions All solid surfaces are assigned thermal boundary conditions, plus information aboutthe burning behavior of the material. Heat and mass transfer to and from solid surfaces is usually handledwith empirical correlations, although it is possible to compute directly the heat and mass transfer whenperforming a Direct Numerical Simulation (DNS).

1.2 What’s New in FDS 5?

FDS 5 differs from previous versions in its treatment of solid boundaries and gas phase combustion. Amongthe more important changes are:

Multi-Step Combustion Previous versions of FDS have assumed only one gas phase reaction. Now,multiple-step reaction schemes are available to describe local extinction, CO production, among var-ious other phenomena. The most important improvements to the combustion model are a more accurateheat release rate calculation, and a better treatment of local flame extinction.

Material Layers Past versions of FDS have assumed that solid boundaries consist of a single homogenouslayer. Now, solid boundaries can be modeled with multiple layers of materials, with each materialspecified via a new namelist group called MATL. This change makes past input files obsolete.

Command Line Format FDS is still run from the command line, but the syntax is slightly different thanin previous versions. See Section 3 for details.

Database Previous versions of FDS used a separate “database” file to store material and reaction parame-ters. This file is no longer available, and now all parameters must be specified within the input file.

Device Descriptions The method used to describe a device and/or sensor (Sprinkler, Heat Detector, Ther-mocouple, etc.) has changed. See Section 13.1 for more information on defining devices and theirproperties. Any device can be used to control sprinkler activation and the creation and removal of ventsor obstacles.

4

Sprinklers The external sprinkler files used in previous versions are no longer used. All information aboutsprinklers and other fire-specific devices are conveyed in the input file. Sprinklers are now defined withthe new method of describing devices mentioned above. See Section 13.1 for more information.

Control Functions A new group of input parameters is available to describe functions that control sprinkleractivation, the creation and removal of vents or obstacles, and code execution (termination or dumpingof restart files). See Section 13.5 for details.

Numerical Mesh Previous versions of FDS used separate input groups to define the numerical grid and thecomputational domain. Now the two groups have been merged into a single, simplified MESH namelistgroup. Namelist groups PDIM and GRID shall no longer be used in the input file. See Section 6.3 formore detail.

Pressure Zones It is possible in FDS 5 to declare individual regions in the computational domain to havebackground pressures different from ambient, allowing for calculations of leakage, fan curves, and soforth. See Section 9.6 for more details.

Stack Effect and Atmospheric Stratification Improvements have been made to better characterize a strat-ified atmosphere and the movement of air in a tall building due to temperature differences between insideand outside.

Adiabatic Surface Temperature A new output quantity has been added to facilitate using FDS output inthermal and mechanical finite element models. See Section 8.2.3 for more information.

Development, Distribution and Formal User Support Starting with FDS 5, an online, open-source de-velopment environment is being used for configuration management (code archiving, revision tracking,bug fixes, user suggestions, and so on). See Section 2.1 for more information.

FDS Verification and Validation Information Starting with FDS 5, more emphasis has been placed onmaintaining a permanent collection of Verification and Validation cases. This improves the quality ofeach FDS update and release, as a standard test suite will now be used to insure that changes made to thesource code do not degrade FDS output. This also provides users with a standard data set to verify theirown installation of FDS and to compare the results that FDS is returning on their system to publisheddata.

5

6

Chapter 2

Getting Started

FDS is a computer program that solves equations that describe the evolution of fire. It is a Fortran programthat reads input parameters from a text file, computes a numerical solution to the governing equations, andwrites user-specified output data to files. Smokeview is a companion program that reads FDS output filesand produces animations on the computer screen. Smokeview has a simple menu-driven interface. FDSdoes not. However, there are various third-party programs that have been developed to generate the text filecontaining the input parameters needed by FDS.

This guide describes how to obtain FDS and Smokeview and how to use FDS. A separate document [2]describes how to use Smokeview. Other tools related to FDS and Smokeview can be found at the web site.

2.1 How to Acquire FDS and Smokeview

Detailed instructions on how to download executables, manuals, source-code and related utilities, can befound on the FDS-SMV Website http://fire.nist.gov/fds. The typical FDS/Smokeview distribu-tion consists of an installation package or compressed archive, which is available for MS Windows, Mac OSX, and Linux. For other operating systems, consult the web site.

If you ever want to keep an older version of FDS and Smokeview, copy the installation directory to someother place so that it is not overwritten during the updated installation.

2.2 Computer Hardware Requirements

FDS requires a fast CPU1 and a substantial amount of random-access memory (RAM) to run efficiently. Forminimum specifications, the system should have a 1 GHz CPU, and at least 512 MB RAM. The CPU speedwill determine how long the computation will take to finish, while the amount of RAM will determine howmany mesh cells can be held in memory. A large hard drive is required to store the output of the calculations.It is not unusual for the output of a single calculation to consume more than 1 GB of storage space.

Most computers purchased within the past few years are adequate for running Smokeview with thecaveat that additional memory (RAM) should be purchased to bring the memory size up to at least 512 MB.This is so the computer can display results without “swapping" to disk. For Smokeview it is also importantto obtain a fast graphics card for the PC used to display the results of the FDS computations.

For Multi-Mesh calculations, the MPI version of FDS will operate over standard 100 Mbps networks.A Gigabit or 1000 Mbps network will further reduce latency and improve data transfer rates between nodes.

1Central Processing Unit

7

2.3 Computer Operating System (OS) and Software Requirements

The goal of making FDS and Smokeview publicly available has been to enable practicing fire protectionengineers to perform fairly sophisticated fire simulations at a reasonable cost. Thus, FDS and Smokeviewhave been designed for computers running Microsoft Windows, Mac OS X, and various implementations ofUnix/Linux.

MS Windows An installation package is available for Windows operating system. It is not recommendedto run FDS/Smokeview under any version of MS Windows released prior to Windows 2000.

Mac OS X A zip archive is available for Intel architectures. Mac OS X 10.4.x or better is recommended,versions of OS X prior to 10.4.x are not officially supported. Users can always download the latestversion of FDS source and compile FDS for other versions of OS X (See Appendix 18 for details).

Linux Pre-compiled executables are available that can be installed in an appropriate directory. Note thatthe installation package is simply an archive and no path variables are set. If the pre-compiled FDSexecutable does not work (usually because of library incompatibilities), the FDS source code can bedownloaded and compiled using a Fortran 90 and C compiler (See Appendix 18 for details). If Smoke-view does not work on the Linux workstation, you can use the Windows version to view FDS output.

Unix There are no pre-compiled versions of FDS for the various flavors of Unix. However, the advice forLinux applies equally as well to Unix.

FDS in Parallel For those wishing to use multiple computers to run a single FDS calculation, MPI (Mes-sage Passing Interface) must be installed on each of the computers within the network that will be usedfor FDS computations.

8

Chapter 3

Running FDS

This chapter describes the procedure to run an FDS calculation. The primary requirement for any calculationis an FDS input file. The creation of an input file is covered in detail in Part II. If you are new to FDS andSmokeview, it is strongly suggested that you start with an existing data file, run it as is, and then make theappropriate changes to the input file for the desired scenario. Sample input files are included as part of thestandard installation. By running a sample case, you become familiar with the procedure, learn how to useSmokeview, and ensure that your computer is up to the task before embarking on learning how to create newinput files.

3.1 Starting an FDS Calculation

FDS can be run from the command prompt, or with a third party Graphical User Interface (GUI). In thediscussion to follow, it is assumed that FDS is being run from the command prompt. FDS can be run on asingle computer, using only one CPU, or it can be run on multiple computers and use multiple CPUs. Forany operating system, there are two FDS executable files. The single CPU Windows executable is calledfds5.exe. The parallel executable is called fds5_mpi.exe. The letters “mpi” in the filename denote MessagePassing Interface (MPI), which will be discussed below.

Note that the input file for both single and parallel versions of FDS are the same. In fact, it is recommendedthat before embarking on parallel processing, you should run your input file in serial mode to ensure that itis properly set up.

3.1.1 Starting an FDS Calculation (Single Processor Version)

Sample input files are provided with the program for new users who are encouraged to first run a samplecalculation before attempting to write an input file. Assuming that an input file called job_name.fds existsin some directory, run the program either in a DOS or Unix command prompt as follows:

MS Windows

Open up a Command Prompt window (click Start, then Run, then type “cmd”), and change directories (“cd”)to where the input file for the case is located, then run the code by typing at the command prompt

fds5 job_name.fds

9

The character string job_name is usually designated within the input file as the CHID. It is recommendedthat the name of the input file and the CHID be the same so that all of the files associated with a givencalculation have a consistent name. The progress of a simulation is indicated by diagnostic output that iswritten out onto the screen. Detailed diagnostic information is automatically written to a file CHID.out,where CHID is a character string, usually the same as job_name, designated in the input file.. Screen outputcan be redirected to a file via the alternative command

fds5 job_name.fds > job_name.err

Note that it is also possible to associate the extention “fds” with the FDS executable directly, thereby makingFDS run by double-clicking on the input file. If you do this, note that error messages will be written to the.out file. Also, if you associate the input file with the FDS executable, be careful not to accidently double-click on the input file when trying to edit it. This action will cause previously generated output files to beover-written.

Mac OS X, Unix, Linux

Depending on the type of installation, you may need to set various path or environment variables in orderto invoke FDS without a full path reference to the executable. The easiest way to do this is via an “alias” inyour shell start-up script. For the example below, it is assumed that fds5 is aliased to its full path name. Youmay also need to “chmod + x” to make the file executable. Once this is done, run FDS from the commandline by typing:

fds5 job_name.fds

The input parameters are read from the file job_name.fds, and error statements and other diagnostics arewritten out to the screen. To run the job in the background:

fds5 job_name.fds >& job_name.err &

Note that in the latter case, the screen output is stored in the file job_name.err and the detailed diagnosticsare saved automatically in a file CHID.out, where CHID is a character string, usually the same as job_name,designated in the input file. It is preferable to run jobs in the background so as to free the console for otheruses.

3.1.2 Starting an FDS Calculation (Multiple Processor Version)

Running FDS across a network using multiple processors and multiple banks of memory (RAM) is moredifficult than running the single processor version. More is required of the user to make the connectionsbetween the machines as seamless as possible. This involves creating accounts for a given user on eachmachine, sharing directories, increasing the speed of the network, making each machine aware of the others,etc. Some of these details are handled by the parallel-processing software, others are not. Undoubtedlythe procedure will be simplified in years to come, but for the moment, parallel-processing is still relativelynew and requires more expertise in terms of understanding both the operating system and the networkconnections of a given set of computers.

FDS uses MPI (Message-Passing Interface) [3] to allow multiple computers to run a single FDS job. Themain idea is that you must break up the FDS domain into multiple meshes, and then the flow field in eachmesh is computed as a different process. Note the subtle difference between these terms – a process does nothave the same meaning as a processor. The process can be thought of as a “task” that you would see in theWindows Task Manager or by executing the “top” command on a Linux/Unix machine. The processor refersto the computer hardware. A single processor may run multiple processes, for example. The computation

10

on a given FDS mesh is thought of as an individual process, and MPI handles the transfer of informationbetween these processes. Usually, each mesh is assigned its own process in a parallel calculation, althoughit is also possible assign multiple meshes to a single process.1 In this way, large meshes can be computed ondedicated processors, while smaller meshes can be clustered together in a single process running on a singleprocessor, without the need for MPI message passing between themselves.

Also note that FDS refers to its meshes by the numbers 1, 2, 3, and so on, whereas MPI refers to itsprocesses by the numbers 0, 1, 2, and so on. Thus, Mesh 1 is assigned to Process 0; Mesh 2 to Process 1,and so on. As a user, you never actually number the meshes or the processes yourself, but error statementsfrom FDS or from MPI might refer to the meshes or processes by number. As an example, if a five meshFDS case is run in parallel, the first printout (usually to the screen unless otherwise directed) is:

Process 4 of 4 is running on fire65Process 3 of 4 is running on fire64Process 2 of 4 is running on fire63Process 0 of 4 is running on fire61Process 1 of 4 is running on fire62Mesh 1 is assigned to Process 0Mesh 2 is assigned to Process 1Mesh 3 is assigned to Process 2Mesh 4 is assigned to Process 3Mesh 5 is assigned to Process 4

This means that 5 processes (numbered 0 to 4) have started on the computers named fire61, fire62, etc., andthat each mesh is being computed as an individual process on the individual computers. Each computer hasits own memory (RAM), and MPI is the protocol by which information is passed from process to processduring the calculation. Note that these computers may have multiple processors, and each processor mayhave multiple “cores.” You have control over how many processes get assigned to each computer, but youmay or may not have control over how the processes are handled by a given computer. That depends onthe operating system and the particular version of MPI. For example, fire62 happens to have two quad-coreprocessors, and all five meshes could have been assigned to run as five processes all on fire62. Whether ornot this is the best strategy is still a subject of research and heavily dependent on the technical specificationsof the OS and hardware.

There are different implementations of MPI, much like there are different Fortran and C compilers. Eachimplementation is essentially a library of subroutines called from FDS that transfer data from one thread toanother across a fast network. The format of the subroutine calls has been widely accepted in the community,allowing different vendors and organizations the freedom to develop better software while working withinan open framework.

The way FDS is executed in parallel depends on which implementation of MPI has been installed. AtNIST, the parallel version of FDS is presently run on Windows PCs connected by the Local Area Network(LAN, 100 Mbps) or on a cluster of Linux PCs linked together with a dedicated, fast (1000 Mbps) network.The Windows computers use MPICH2, a free implementation of MPI from Argonne National Laboratory,USA.

MPICH2

With MPICH2, a parallel FDS calculation can be invoked either from the command line or by using aGraphical User Interface (GUI). After the MPICH2 libraries are installed on each computer and the neces-sary directories are shared, FDS is run using the command issued from one of the computers

1Assigning multiple meshes to a single process was introduced in FDS version 5.3.

11

mpiexec -file config.txt

where config.txt is a text file containing the name and location of the FDS executable, name of the FDS inputfile, the working directory, and the names of the various computers that are to run the job. For example, theconfig.txt file might look like this for a job run at NIST with computers named fire_1, fire_2, and fire_3:

exe \\fire_1.nist.gov\NIST\FDS\fds5_mpi.exe job_name.fdsdir \\fire_1.nist.gov\Projects\hostsfire_1.nist.gov 2fire_2.nist.gov 1fire_3.nist.gov 2

The numbers following the “host” machines represent the number of threads to run on that particular ma-chine. In this example, 5 threads are run for an FDS calculation that has 5 meshes. The exe and dir

directories need to be shared, with the latter having read and write permissions.

All the computers must be able to access the executable and the working directory on fire_1. This isachieved under Windows by “sharing.” Under Unix/Linux and OS X, the process involves cross-mountingthe file systems of the various machines.

LAM-MPI

On the Linux cluster in the Building and Fire Research Lab at NIST, LAM-MPI, a free implementation fromIndiana University, is installed.2 With LAM/MPI, the computers to be used are linked prior to the actualexecution of FDS with a separate command called a “lamboot.” FDS is then run using the command

mpirun -np 5 fds5_mpi job_name.fds

where the 5 indicates that 5 processors are to be used. In this case, the executable fds5_mpi is located in theworking directory. To make the process run in the background

mpirun -np 5 fds5_mpi job_name.fds >& job_name.err &

The file job_name.err contains what is normally printed out to the screen.

Note that there are several other implementations of MPI, some free, some not. Support for the softwarevaries, thus FDS has been designed to run under any of the more popular versions without too much userintervention. However, keep in mind that parallel processing is a relatively new area of computer science,and there are bound to be painful growth spurts in the years ahead.

3.2 Monitoring Progress

Diagnostics for a given calculation are written into a file called CHID.out. The CPU usage and simulationtime are written here, so you can see how far along the program has progressed. At any time during acalculation, Smokeview can be run and the progress can be checked visually. To stop a calculation beforeits scheduled time, either kill the process, or preferably create a file in the same directory as the output files

2http://www.lam-mpi.org

12

called CHID.stop. The existence of this file stops the program gracefully, causing it to dump out the latestflow variables for viewing in Smokeview.

Since calculations can be hours or days long, there is a restart feature in FDS. Details of how to use thisfeature are given in Section 6.4.2. Briefly, specify at the beginning of calculation how often a “restart” fileshould be saved. Should something happen to disrupt the calculation, like a power outage, the calculationcan be restarted from the time the last restart file was saved.

It is also possible to control the stop time and the dumping of restart files by using control functions asdescribed in Section 13.5.

13

14

Chapter 4

User Support

It is not unusual over the course of a project to run into various problems, some related to FDS, some relatedto your computer. FDS is not a typical PC application. It is a serious calculation that pushes your computer’sprocessor and memory to its limits. In fact, there are no hardwired bounds within FDS that prevent youfrom starting a calculation that is too much for your hardware. Even if your machine has adequate memory(RAM), you can still easily set up calculations that can require weeks or months to complete. It is difficultto predict at the start of a simulation just how long and how much memory will be required. Learn how tomonitor the resource usage of your computer. Start with small calculations and build your way up.

Although many features in FDS are fairly mature, there are many that are not. FDS is used for practicalengineering applications, but also for research in fire and combustion. As you become more familiar with thesoftware, you will inevitably run into areas that are of current research interest. Indeed, burning a roomfulof ordinary furniture is one of the most challenging applications of the model. So be patient, and learn todissect a given scenario into its constitutive parts. For example, do not attempt to simulate a fire spreadingthrough an entire floor of a building unless you have simulated the burning of the various combustibles withrelatively small calculations.

Along with the FDS User’s Guide, there are resources available on the Internet. These include an “IssueTracker,” that allows you to report bugs, request new features, and ask specific clarifying questions, and“Group Discussions,” which support more general topics than just specific problems. Before using these on-line resources, it is important to first try to solve your own problems by performing simple test calculations,or debugging your input file. The next few sections provide a list of error statements and suggestions onhow to solve problems.

4.1 The Version Number

If you encounter problems with FDS, it is crucial that you submit, along with a description of the problem,the FDS version number. Each release of FDS comes with a version number like 5.2.6, where the firstnumber is the major release, the second is the minor release, and the third is the maintenance release.Major releases occur every few years, and as the name implies dramatically change the functionality of themodel. Minor releases occur every few months, and may cause minor changes in functionality. Releasenotes can help you decide whether the changes should effect the type of applications that you typically do.Maintenance releases are just bug fixes, and should not affect code functionality. To get the version number,just type the executable at the command prompt:

fds5

15

and the relevant information will appear, along with a date of compilation (useful to you) and a so-calledSVN number (useful to us). The SVN number refers to the Subversion repository number of the sourcecode. It allows us to go back in time and recover the exact source code files that were used to build thatexecutable.

Simply get in the habit of checking the version number of your executable, periodically checking fornew releases which might already have addressed your problem, and telling us what version you are usingif you report a problem.

4.2 Common Error Statements

An FDS calculation may end before the specified time limit. Following is a list of common error statementsand how to diagnose the problems:

Input File Errors: The most common errors in FDS are due to mis-typed input statements. These errorsresult in the immediate halting of the program and a statement like, “ERROR: Problem with the HEADline.” For these errors, check the line in the input file named in the error statement. Make sure theparameter names are spelled correctly. Make sure that a / (forward slash) is put at the end of eachnamelist entry. Make sure that the right type of information is being provided for each parameter, likewhether one real number is expected, or several integers, or whatever. Make sure there are no non-ASCIIcharacters being used, as can sometimes happen when text is cut and pasted from other applications orword-processing software. Make sure zeros are zeros and O’s are O’s. Make sure 1’s are not !’s. Makesure apostrophes are used to designate character strings. Make sure the text file on a Unix/Linux machinewas not created on a DOS machine, and vice versa. Make sure that all the parameters listed are stillbeing used – new versions of FDS often drop or change parameters to force you to re-examine old inputfiles.

Numerical Instability Errors: It is possible that during an FDS calculation the flow velocity at some loca-tion in the domain can increase due to numerical error causing the time step size to decrease to a pointwhere logic in the code decides that the results are unphysical and stops the calculation with an errormessage in the file CHID.out. In these cases, FDS ends by dumping out one final Plot3D file givingthe user some means by which to see where the error is occurring within the computational domain.Usually, a numerical instability can be identified by fictitiously large velocity vectors emanating from asmall region within the domain. Common causes of such instabilities are mesh cells that have an aspectratio larger than 2 to 1, high speed flow through a small opening, a sudden change in the heat releaserate, or any number of sudden changes to the flow field. There are various ways to solve the problem,depending on the situation. Try to diagnose and fix the problem before reporting it. It is difficult foranyone but the originator of the input file to diagnose the problem.

Inadequate Computer Resources: The calculation might be using more RAM than the machine has, orthe output files could have used up all the available disk space. In these situations, the computer may ormay not produce an intelligible error message. Sometimes the computer is just unresponsive. It is yourresponsibility to ensure that the computer has adequate resources to do the calculation. Remember, thereis no limit to how big or how long FDS calculations can be – it depends on the resources of the computer.For any new simulation, try running the case with a modest-sized mesh, and gradually make refinementsuntil the computer can no longer handle it. Then back off somewhat on the size of the calculation so thatthe computer can comfortably run the case. Trying to run with 90 % to 100 % of computer resources isrisky. In fact, for a typical 32 bit Windows PC with 4 GB RAM, only 2 GB will be available to FDS,based on user feedback. If you want to run bigger cases, consider buying a computer with a 64 bitoperating system or break up the calculation into multiple meshes and use parallel processing.

16

Run-Time Errors: An error occurs either within the computer operating system or the FDS program. Anerror message is printed out by the operating system of the computer onto the screen or into the diag-nostic output file. This message is most often unintelligible to most people, including the programmers,although occasionally one might get a small clue if there is mention of a specific problem, like “stackoverflow,” “divide by zero,” or “file write error, unit=...” These errors may be caused by a bug in FDS,for example if a number is divided by zero, or an array is used before it is allocated, or any numberof other problems. Before reporting the error to the Issue Tracker, try to systematically simplify theinput file until the error goes away. This process usually brings to light some feature of the calculationresponsible for the problem and helps in the debugging.

File Writing Errors: Occasionally, especially on Windows machines, FDS fails because it is not permittedto write to a file. A typical error statement reads:

forrtl: severe (47): write to READONLY file, unit 8598, file C:\Users\...\

The unit, in this case 8598, is just a number that FDS has associated with one of the output files. If thiserror occurs just after the start of the calculation, you can try adding the phraseFLUSH_FILE_BUFFERS=.FALSE.

on the DUMP line of the input file (see Section 14.1). This will prevent FDS from attempting to flushthe contents of the internal buffers, something it does to make it possible to view the FDS output inSmokeview during the FDS simulation.

Poisson Initialization: Sometimes at the very start of a calculation, an error appears stating that there is aproblem with the “Poisson initialization.” The equation for pressure in FDS is known as the Poissonequation. The Poisson solver consists of large system of linear equations that must be initialized at thestart of the calculation. Most often, an error in the initialization step is due to a mesh IJK dimensionbeing less than 4 (except in the case of a two-dimensional calculation). It is also possible that somethingis fundamentally wrong with the coordinates of the computational domain. Diagnose the problem bychecking the MESH lines in the input file.

4.3 Support Requests and Bug Tracking

Because FDS development is on-going, problems will inevitably occur with various routines and features.The developers need to know if a certain feature is not working, and reporting problems is encouraged.However, the problem must be clearly identified. The best way to do this is to simplify the input file asmuch as possible so that the bug can be diagnosed. Also, limit the bug reports to those features that clearlydo not work. Physical problems such as fires that do not ignite, flames that do not spread, etc., may berelated to the mesh resolution or scenario formulation and need to be investigated first by the user beforebeing reported. If an error message originates from the operating system as opposed to FDS, first investigatesome of the more obvious possibilities, such as memory size, disk space, etc.

If that does not solve the problem, report the problem with as much information about the error messageand circumstances related to the problem. The input file should be simplified as much as possible so that thebug occurs early in the calculation. Attach the simplified input file if necessary, following the instructionsprovided at the web site. In this way, the developers can quickly run the problem input file and hopefullydiagnose the problem.

NOTE: Reports of specific problems, feature requests and enhancements should be posted to the IssueTracker and not the Discussion Group.

17

4.4 Known Issues

As a result of collecting feedback from FDS users over roughly a decade, we have identified a number offeatures in FDS that can be problematic for a variety of reasons. Table 4.1 lists these features that are eitherunder development, or that have been cited a number of times by users who have observed spurious behavior,inconsistent or inaccurate results, fragility, and so on. For those interested in FDS model development, thislist is ripe for further research. For those who use FDS for engineering applications, these may be featuresto avoid until they can be made more reliable and robust.

18

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19

20

Part II

Writing an FDS Input File

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Chapter 5

The Basic Structure of an Input File

5.1 Naming the Job

The operation of FDS is based on a single input text1 file containing parameters organized into namelist 2

groups. The input file provides FDS with all of the necessary information to describe the scenario. Theinput file is saved with a name such as job_name.fds, where job_name is any character string that helps toidentify the simulation. If this same string is repeated under the HEAD namelist group within the input file,then all of the output files associated with the calculation will then have this common name.

There should be no blank spaces in the job name. Instead use the underscore character to representa space. Using an underscore characters instead of a space also applies to the general practice of namingdirectories on your system.

Be aware that FDS will simply over-write the output files of a given case if its assigned name is thesame. This is convenient when developing an input file because you save on disk space. Just be careful notto overwrite a calculation that you want to keep.

5.2 Namelist Formatting

Parameters are specified within the input file by using namelist formatted records. Each namelist recordbegins with the ampersand character “&” followed immediately by the name of the namelist group, then acomma-delimited list of the input parameters, and finally a forward slash “/”. For example, the line

&DUMP NFRAMES=1800, DT_HRR=10., DT_DEVC=10., DT_PROF=30. /

sets various values of parameters contained in the DUMP namelist group. The meanings of these variousparameters will be explained in subsequent chapters. The namelist records can span multiple lines in theinput file, but just be sure to end the record with a “/” or else the data will not be understood. Do not addanything to a namelist line other than the parameters and values appropriate for that group. Otherwise, FDSwill stop immediately upon execution.

Parameters within a namelist record can be separated by either commas, spaces, or line breaks. It is agood idea to use commas or line breaks, and never use tab stops. Some machines do not recognize the spacesor the length of the tab stops. Comments and notes can be written into the file so long as nothing comesbefore the & except a space and nothing comes between the ampersand & and the slash / except appropriateparameters corresponding to that particular namelist group.

1ASCII – American Standard Code for Information Interchange2A namelist is a Fortran input record.

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The parameters in the input file can be integers (T_END=5400), real numbers (CO_YIELD=0.008),groups of real numbers or integers (XYZ=6.04,0.28,3.65) or (IJK=90,36,38), character strings:

CHID=’WTC_05_v5’

groups of character strings:SURF_IDS=’burner’,’STEEL’,’BRICK’

or logical parameters:POROUS_FLOOR=.FALSE.

A logical parameter is either .TRUE. or .FALSE. – the periods are a Fortran convention. Character stringsthat are listed in this User’s Manual must be copied exactly as written – the code is case sensitive andunderscores do matter.

Most of the input parameters are simply real or integer scalars, like DT=0.02, but sometimes the in-puts are multidimensional arrays. For example, when describing a particular solid surface, you need toexpress the mass fractions of multiple materials that are to be found in multiple layers. The input arrayMATL_MASS_FRACTION(IL,IC) is intended to convey to FDS the mass fraction of component IC of layerIL. For example, if the mass fraction of the second material of the third layer is 0.5, then write

MATL_MASS_FRACTION(3,2)=0.5

To enter more than one mass fraction, use this notation:MATL_MASS_FRACTION(1,1:3)=0.5,0.4,0.1

which means that the first three materials of layer 1 have mass fractions of 0.5, 0.4, and 0.1, respectively.The notation 1:3 means array element 1 through 3, inclusive.

Note that character strings can be enclosed either by apostrophes or quotation marks. Be careful not to createthe input file by pasting text from something other than a simple text editor, in which case the punctuationmarks may not transfer properly into the text file.

Note that depending on compiler and operating system, some text file encodings may not work on all sys-tems. If file reading errors occur and no typographical errors can be found in the input file, try saving theinput file using a different encoding. It does not appear that current Fortran compilers support the UTF-8encoding standard for reading Namelist inputs.

5.3 Input File Structure

In general, the namelist records can be entered in any order in the input file, but it is a good idea to organizethem in some systematic way. Typically, general information is listed near the top of the input file, anddetailed information, like obstructions, devices, and so on, are listed below. FDS scans the entire input fileeach time it processes a particular namelist group. With some text editors, it has been noticed that the lastline of the file is often not read by FDS because of the presence of an “end of file” character. To ensure thatFDS reads the entire input file, add

&TAIL /

as the last line at the end of the input file. This completes the file from &HEAD to &TAIL. FDS does not evenlook for this last line. It just forces the “end of file” character past relevant input.

Another general rule of thumb when writing input files is to only add to the file parameters that are tochange from their default value. That way, you can more easily distinguish between what you want and what

24

FDS wants. Add comments liberally to the file, so long as these comments do not fall within the namelistrecords.

The general structure of an input file is shown below, with many lines of the original validation inputfile3 removed for clarity.

&HEAD CHID='WTC_05_v5', TITLE='WTC Phase 1, Test 5, FDS version 5' /&MESH IJK=90,36,38, XB=-1.0,8.0,-1.8,1.8,0.0,3.82 /&TIME T_END=5400. /&MISC SURF_DEFAULT='MARINITE BOARD', TMPA=20., POROUS_FLOOR=.FALSE. /&DUMP NFRAMES=1800, DT_HRR=10., DT_DEVC=10., DT_PROF=30. /

&REAC ID = 'HEPTANE TO CO2'FYI = 'Heptane, C_7 H_16'C = 7.H = 16.CO_YIELD = 0.008 /SOOT_YIELD = 0.015 /

&OBST XB= 3.5, 4.5,-1.0, 1.0, 0.0, 0.0, SURF_ID='STEEL FLANGE' / Fire Pan...&SURF ID = 'STEEL FLANGE'

COLOR = 'BLACK'MATL_ID = 'STEEL'BACKING = 'EXPOSED'THICKNESS = 0.0063 /

...&VENT MB='XMIN',SURF_ID='OPEN' /...&SLCF PBY=0.0, QUANTITY='TEMPERATURE', VECTOR=.TRUE. /...&BNDF QUANTITY='GAUGE HEAT FLUX' /...&DEVC XYZ=6.04,0.28,3.65, QUANTITY='oxygen', ID='EO2_FDS' /...&TAIL / End of file.

It is strongly recommended that when looking at a new scenario, first select a pre-written input file thatresembles the case, make the necessary changes, then run the case at fairly low resolution to determine if thegeometry is set up correctly. It is best to start off with a relatively simple file that captures the main featuresof the problem without getting tied down with too much detail that might mask a fundamental flaw in thecalculation. Initial calculations ought to be meshed coarsely so that the run times are less than an hour andcorrections can easily be made without wasting too much time. As you learn how to write input files, youwill continually run and re-run your case as you add in complexity.

Table 5.1 provides a quick reference to all the namelist parameters and where you can find the referenceto where it is introduced in the document and the table containing all of the keywords for each group.

3The actual input file, WTC_05_v5.fds, is part of the FDS Validation Suite

25

Table 5.1: Namelist Group Reference Table

Group Name Namelist Group Description Reference Section Parameter TableBNDF Boundary File Output 14.2.4 15.1CTRL Control Function Parameters 13.5 15.3DEVC Device Parameters 13.1 15.4DUMP Output Parameters 14.1 15.5HEAD Input File Header 6.1 15.6HOLE Obstruction Cutout 7.3 15.7INIT Initial Condition 6.5 15.8ISOF Isosurface File Output 14.2.5 15.9MATL Material Property 8.3 15.10MESH Mesh Parameters 6.3 15.11MISC Miscellaneous 6.4 15.12MULT Multiplier Parameters 7.2.2 15.13OBST Obstruction 7.2 15.14PART Lagrangian Particle 12 15.15PRES Pressure Solver Parameters 6.6 15.16PROF Profile Output 14.2.2 15.17PROP Device Property 13.3 15.18RADI Radiation 11.3 15.19RAMP Ramp Profile 10 15.20REAC Reactions 11.1 15.21SLCF Slice File Output 14.2.3 15.22SPEC Species Parameters 11.2 15.23SURF Surface Properties 7.1 15.24TIME Simulation Time 6.2 15.26TRNX Mesh Stretching 6.3.5 15.27VENT Vent Parameters 7.4 15.28ZONE Pressure Zone Parameters 9.6 15.29

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Chapter 6

Setting the Bounds of Time and Space

6.1 Naming the Job: The HEAD Namelist Group (Table 15.6)

The first thing to do when setting up an input file is to give the job a name. The name of the job is importantbecause often a project involves numerous simulations in which case the names of the individual simulationscan help organize the effort. The namelist group HEAD contains two parameters, as in this example:

&HEAD CHID='WTC_05_v5', TITLE='WTC Phase 1, Test 5, FDS version 5' /

CHID is a string of 30 characters or less used to tag the output files. If, for example, CHID=’WTC_05_v5’,it is convenient to name the input data file WTC_05_v5.fds so that the input file can be associatedwith the output files. No periods or spaces are allowed in CHID because the output files are tagged withsuffixes that are meaningful to certain computer operating systems.

TITLE is a string of 60 characters or less that describes the simulation. It is simply descriptive text that ispassed to various output files.

6.2 Simulation Time: The TIME Namelist Group (Table 15.26)

TIME is the name of a group of parameters time define the time duration of the simulation and the initialtime step used to advance the solution of the discretized equations.

6.2.1 Basics

Usually, only the duration of the simulation is required on this line, via the parameter T_END. The default is1 s. Note: the older TWFIN will still work but it has been deprecated, it is recommended that T_END be usednow instead.

For example, the following line will instruct FDS to run the simulation for 5400 seconds.

&TIME T_END=5400. /

If T_END is set to zero, only the set-up work is performed, allowing you to quickly check the geometry inSmokeview.

If you want the time line to start at a number other than zero, you can use the parameter T_BEGINto specify the time written to file for the first time step. This would be useful for matching time lines ofexperimental data or video recordings.

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Time-based RAMPs are evaluated using the actual time if the RAMP activation time is the same as T_BEGIN;otherwise, they are evaluated using the time from when the RAMP activates. Therefore, if you are settingT_BEGIN in order to test a time-based CTRL or DEVC that is ultimately linked to a RAMP, then you should setT_BEGIN to be slightly less than the time the RAMP will activate. For example if you are testing a VENT thatis to open at 10 s whose SURF_ID uses a RAMP, T_BEGIN should be set slightly less than 10 s.

6.2.2 Special Topic: Controlling the Time Step

The initial time step size can be specified with DT. This parameter is normally set automatically by dividingthe size of a mesh cell by the characteristic velocity of the flow. During the calculation, the time step isadjusted so that the CFL (Courant, Friedrichs, Lewy) condition is satisfied. The default value of DT is5(δxδyδz)

13 /√

gH s, where δx, δy, and δz are the dimensions of the smallest mesh cell, H is the height ofthe computational domain, and g is the acceleration of gravity. Note that by default the time step is neverallowed to increase above its initial value. To allow this to happen, set RESTRICT_TIME_STEP=.FALSE.

If something sudden is to happen right at the start of a simulation, like a sprinkler activation, it is a good ideato set the initial time step to avoid a numerical instability caused by too large a time step. Experiment withdifferent values of DT by monitoring the initial time step sizes recorded in the output file job_name.out.

One additional parameter in the TIME group is SYNCHRONIZE, a logical flag (.TRUE. or .FALSE.) indi-cating that in a multi-mesh computation the time step for each mesh should be the same, thus ensuring thateach mesh is processed each iteration. More details can be found in Section 6.3.3. The default value ofSYNCHRONIZE is .TRUE.

Finally, if you want to prevent FDS from automatically changing the time step, setLOCK_TIME_STEP=.TRUE.

on the TIME line, in which case the specified time step, DT, will not be adjusted. This parameter is intendedfor diagnostic purposes only, for example, timing program execution. It can lead to numerical instabilitiesif the initial time step is set too high.

6.2.3 Special Topic: Steady-State Applications

Occasionally, there are applications in which only the steady-state solution (in a time-averaged sense) isdesired. However, the time necessary to heat the walls to steady-state can make the cost of the calculationprohibitive. In these situations, if you specify a TIME_SHRINK_FACTOR of, say, 10, the specific heats ofthe various materials is reduced by a factor of 10, speeding up the heating of these materials roughly by 10.An example of an application where this parameter is handy is a validation experiment where a steady heatsource warms up a compartment to a nearly equilibrium state at which point time-averaged flow quantitiesare measured.

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6.3 Computational Meshes: The MESH Namelist Group (Table 15.11)

All FDS calculations must be performed within a domain that is made up of rectilinear volumes calledmeshes. Each mesh is divided into rectangular cells, the number of which depends on the desired resolutionof the flow dynamics. MESH is the namelist group that defines the computational domain.

6.3.1 Basics

A mesh is a single right parallelepiped, i.e. a box. The coordinate system within a mesh conforms to theright hand rule. The origin point of a mesh is defined by the first, third and fifth values of the real numbersextuplet, XB, and the opposite corner is defined by the second, fourth and sixth values. For example,

&MESH IJK=10,20,30, XB=0.0,1.0,0.0,2.0,0.0,3.0 /

defines a mesh that spans the volume starting at the origin and extending 1 m in the positive x direction, 2 min the positive y direction, and 3 m in the positive z direction. The mesh is subdivided into uniform cells viathe parameter IJK. In this example, the mesh is divided into 10 cm cubes. If it is desired that the mesh cellsin a particular direction not be uniform in size, then the namelist groups TRNX, TRNY and/or TRNZ may beused to alter the uniformity of the mesh (See Section 6.3.5).

Any obstructions or vents that extend beyond the boundary of the mesh are cut off at the boundary. Thereis no penalty for defining objects outside of the mesh, and these objects will not appear in Smokeview.

Note that it is best if the mesh cells resemble cubes, that is, the length, width and height of the cells oughtto be roughly the same.

Because an important part of the calculation uses a Poisson solver based on Fast Fourier Transforms (FFTs)in the y and z directions, the second and third dimensions of the mesh should each be of the form 2l 3m 5n,where l, m and n are integers. For example, 64 = 26, 72 = 2332 and 108 = 2233 are good mesh cell divisions,but 37, 99 and 109 are not. The first number of mesh cell divisions (the I in IJK) does not use FFTs andneed not be given as a product of small numbers. However, you should experiment with different values ofdivisions to ensure that those that are ultimately used do not unduly slow down the calculation.

Here is a list of numbers between 1 and 1024 that can be factored down to 2’s, 3’s and 5’s:

2 3 4 5 6 8 9 10 12 15 16 18 20 24 2527 30 32 36 40 45 48 50 54 60 64 72 75 80 8190 96 100 108 120 125 128 135 144 150 160 162 180 192 200

216 225 240 243 250 256 270 288 300 320 324 360 375 384 400405 432 450 480 486 500 512 540 576 600 625 640 648 675 720729 750 768 800 810 864 900 960 972 1000 1024

6.3.2 Two-Dimensional and Axially-Symmetric Calculations

The governing equations solved in FDS are written in terms of a three dimensional Cartesian coordinatesystem. However, a two dimensional Cartesian or two dimensional cylindrical (axially-symmetric) calcu-lation can be performed by setting the J in the IJK triplet to 1 on the MESH line. For axial symmetry,add CYLINDRICAL=.TRUE. to the MESH line, and the coordinate x is then interpreted as the radial coordi-nate r. No boundary conditions should be set at the planes y = YMIN=XB(3) or y = YMAX=XB(4), nor atr = XMIN=XB(1) in an axially-symmetric calculation in which r = XB(1)=0. For better visualizations, thedifference between XB(4) and XB(3) should be small so that the Smokeview rendering appears to be in2-D. An example of an axially-symmetric helium plume (helium_2d) is given in Section 6.4.4.

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6.3.3 Multiple Meshes and Parallel Processing

Figure 6.1: An example of a multiple-mesh geometry.

The term “multiple meshes” means that the computational domain consists of more than one computa-tional mesh, usually connected although this is not required. If more than one mesh is used, there shouldbe a MESH line for each. The order in which these lines are entered in the input file matters. In general, themeshes should be entered from finest to coarsest. FDS assumes that a mesh listed first in the input file hasprecedence over a mesh listed second if the two meshes overlap. Meshes can overlap, abut, or not touchat all. In the last case, essentially two separate calculations are performed with no communication at allbetween them. Obstructions and vents are entered in terms of the overall coordinate system and need notapply to any one particular mesh. Each mesh checks the coordinates of all the geometric entities and decideswhether or not they are to be included.

To run FDS in parallel using MPI (Message Passing Interface), you must break up the computationaldomain into multiple meshes so that the workload can be divided among the available processors. In general,it is better to run multiple mesh cases with the parallel version of FDS if you have the computers available,but be aware that two computers will not necessarily finish the job in half the time as one. For the parallelversion to work well, there has to be a comparable number of cells in each mesh, or otherwise most of thecomputers will sit idle waiting for the one with the largest mesh to finish processing each time step. Youcan use multiple meshes even when running the serial version of FDS, in which case one CPU will seriallyprocess each mesh, one by one. Why do this? For one, if you setSYNCHRONIZE=.FALSE.

on the TIME line, then in each mesh, the governing equations will be solved with a time step based on theflow speed within that particular mesh. Because each mesh can have different time steps, this technique cansave CPU time by requiring relatively coarse meshes to be updated only when necessary. Coarse meshesare best used in regions where temporal and spatial gradients of key quantities are small or unimportant. Be

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aware, however, that unsynchronized time steps are more likely to lead to numerical instabilities.By default, the time steps in each mesh are synchronized. With this setting, all meshes are active each

iteration. For a single-processor, multiple mesh calculation, this strategy reduces and may even eliminateany benefit seen by using multiple meshes. However, in a parallel calculation, if a particular mesh is inactiveduring an iteration because it is not ready to be updated, then the processor assigned to that mesh is alsoinactive. Forcing the mesh to be updated with a smaller than ideal time step does not cost anything sincethat processor would have been idle anyway. The benefit is that there is a tighter connection betweenmeshes. It is also possible to synchronize the time step in only a select set of meshes. To do this, addSYNCHRONIZE=.TRUE. to the appropriate MESH lines and then add SYNCHRONIZE=.FALSE. to the TIMEline.

Usually in a multi-mesh calculation, each mesh is assigned its own process, and each process its ownprocessor. However, it is possible to assign more than one mesh to a single process, and it is possible toassign more than one process to a single processor. Consider a case that involves six meshes:

&MESH ID='mesh1', IJK=..., XB=..., MPI_PROCESS=0 /&MESH ID='mesh2', IJK=..., XB=..., MPI_PROCESS=1 /&MESH ID='mesh3', IJK=..., XB=..., MPI_PROCESS=1 /&MESH ID='mesh4', IJK=..., XB=..., MPI_PROCESS=2 /&MESH ID='mesh5', IJK=..., XB=..., MPI_PROCESS=3 /&MESH ID='mesh6', IJK=..., XB=..., MPI_PROCESS=3 /

The parameter MPI_PROCESS instructs FDS to assign that particular mesh to the given process. In this case,only four processes are to be started, numbered 0 through 3. Note that the processes need to be invoked inascending order, starting with 0. Why would you do this? Suppose you only have four processors availablefor this job. By starting only four processes instead of six, you can save time because ‘mesh2’ and ‘mesh3’can communicate directly with each other without having to transmit data using MPI calls over the network.Same goes for ‘mesh5’ and ‘mesh6’. In essence, it is as if these mesh pairs are neighbors and need not sendmail to each other via the postal system. The letters can just be walked next door.

6.3.4 Mesh Alignment

Whether the calculation is to be run on a single processor, or on multiple processors, the rules of prescribingmultiple meshes are similar, with some issues to keep in mind. The most important rule of mesh alignmentis that abutting cells ought to have the same cross sectional area, or integral ratios, as shown in Fig. 6.2.The requirement of integral mesh alignment is new starting with FDS version 5.1. It is a more restrictiverequirement than in previous versions because it was becoming too difficult to maintain complete flexibilityin alignment while trying to improve the accuracy of the methodology. The following rules of thumb shouldalso be followed when setting up a multiple mesh calculation:

• Avoid putting mesh boundaries where critical action is expected, especially fire. Sometimes fire spreadfrom mesh to mesh cannot be avoided, but if at all possible try to keep mesh interfaces relatively free ofcomplicated phenomena since the exchange of information across mesh boundaries is not yet as accurateas cell to cell exchanges within one mesh.

• In general, there is little advantage to overlapping meshes because information is only exchanged atexterior boundaries. This means that a mesh that is completely embedded within another receives infor-mation at its exterior boundary, but the larger mesh receives no information from the mesh embeddedwithin. Essentially, the larger, usually coarser, mesh is doing its own simulation of the scenario andis not affected by the smaller, usually finer, mesh embedded within it. Details within the fine mesh,

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This is the ideal kind of mesh tomesh alignment.

This is allowed so long as thereare an integral number of finecells abutting each coarse cell.

This is allowed, but of ques-tionable value.

This is no longer allowed inFDS 5.1 and higher.

Figure 6.2: Rules governing the alignment of meshes.

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especially related to fire growth and spread, may not be picked up by the coarse mesh. In such cases,it is preferable to isolate the detailed fire behavior within one mesh, and position coarser meshes at theexterior boundary of the fine mesh. Then the fine and coarse meshes mutually exchange information.

• Be careful when using the shortcut convention of declaring an entire face of the domain to be an OPEN

vent. Every mesh takes on this attribute. See Section 7.4 for more details.

• If a planar obstruction is close to where two meshes abut, make sure that each mesh “sees” the obstruc-tion. If the obstruction is even a millimeter outside of one of the meshes, that mesh does not account forit, in which case information is not transferred properly between meshes.

Accuracy of the Parallel Calculation

Experiment with different mesh configurations using relatively coarse mesh cells to ensure that informationis being transferred properly from mesh to mesh. There are two issues of concern. First, does it appear thatthe flow is being badly affected by the mesh boundary? If so, try to move the mesh boundaries away fromareas of activity. Second, is there too much of a jump in cell size from one mesh to another? If so, considerwhether the loss of information moving from a fine to a coarse mesh is tolerable.

Efficiency of the Parallel Calculation

When running a case with multiple meshes in parallel, the efficiency of the calculation can be checkedas follows: (1) Set SYNCHRONIZE=.TRUE. on the TIME line, (2) Let the program run several hundredtime steps, (3) Calculate the difference in wall clock time between two 100 iteration print outs in the fileCHID.out (see Section 19.1). Divide the time difference by 100. This is the average elapsed wall clock timeper time step, (4) Look at the CPU/step for each mesh. The largest value should be less than, but close to,the average elapsed wall clock time. The efficiency of the parallel calculation is the maximum CPU/step

divided by the average wall clock time per step. If this number is between 90 % and 100 %, the parallelcode is working well.

6.3.5 Mesh Stretching: The TRNX, TRNY and/or TRNZ Namelist Groups (Table 15.27)

By default the mesh cells that fill the computational domain are uniform in size. However, it is possibleto specify that the cells be non-uniform in one or two of the three coordinate directions. For a given co-ordinate direction, x, y or z, a function can be prescribed that transforms the uniformly-spaced mesh to anon-uniformly spaced mesh. Be careful with mesh transformations! If you shrink cells in one region youmust stretch cells somewhere else. When one or two coordinate directions are transformed, the aspect ratioof the mesh cells in the 3D mesh will vary. To be on the safe side, transformations that alter the aspect ratioof cells beyond 2 or 3 should be avoided. Keep in mind that the large eddy simulation technique is basedon the assumption that the numerical mesh should be fine enough to allow the formation of eddies that areresponsible for the mixing. In general, eddy formation is limited by the largest dimension of a mesh cell,thus shrinking the mesh resolution in one or two directions may not necessarily lead to a better simulationif the third dimension is large.

Transformations, in general, reduce the efficiency of the computation, with two coordinate transformationsimpairing efficiency more than a transformation in one coordinate direction. Experiment with differentmeshing strategies to see how much of a penalty you will pay.

Here is an example of how to do a mesh transformation. Suppose your mesh is defined

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Figure 6.3: Piecewise-Linear Mesh Transforma-tion.

Figure 6.4: Polynomial Mesh Transformation.

&MESH IJK=15,10,20, XB=0.0,1.5,1.2,2.2,3.2,5.2 /

and you want to alter the uniform spacing in the x direction. First, refer to the figures above. You needto define a function x = f (ξ) that maps the uniformly-spaced Computational Coordinate (CC) 0 ≤ ξ ≤ 1.5to the Physical Coordinate (PC) 0 ≤ x ≤ 1.5. The function has three mandatory constraints: it must bemonotonic (always increasing), it must map ξ = 0 to x = 0, and it must map ξ = 1.5 to x = 1.5. The defaulttransformation function is f (ξ) = ξ for a uniform mesh, but you need not do anything in this case.

Two types of transformation functions are allowed. The first, and simplest, is a piecewise-linear func-tion. Figure 6.3 gives an example of a piecewise-linear transformation. The graph indicates how 15 uni-formly spaced mesh cells along the horizontal axis are transformed into 15 non-uniformly spaced cells alongthe vertical axis. In this case, the function is made up of straight line segments connecting points (CC,PC),in increasing order, as specified by the following lines in the input file:

&TRNX CC=0.30, PC=0.50, MESH_NUMBER=2 /&TRNX CC=1.20, PC=1.00, MESH_NUMBER=2 /

The parameter CC refers to the Computational Coordinate, ξ, located on the horizontal axis; PC is thePhysical Coordinate, x, located on the vertical axis. The slopes of the line segments in the plot indicatewhether the mesh is being stretched (slopes greater than 1) or shrunk (slopes less than 1). The tricky partabout this process is that you usually have a desired shrinking/stretching strategy for the Physical Coordinateon the vertical axis, and must work backwards to determine what the corresponding points should be for theComputational Coordinate on the horizontal axis. Note that the above transformation is applied to the secondmesh in a multiple mesh job.

The second type of transformation is a polynomial function whose constraints are of the form

dn f (CC)dξn = PC

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Figure 6.4 gives an example of a polynomial transformation, for which the parameters are specified (assum-ing that this is the third mesh):

&TRNX IDERIV=0, CC=0.75, PC=0.75, MESH_NUMBER=3 /&TRNX IDERIV=1, CC=0.75, PC=0.50, MESH_NUMBER=3 /

which correspond to the constraints f (0.75) = 0.75 and d fdξ

(0.75) = 0.5, or, in words, the function maps 0.75into 0.75 and the slope of the function at ξ = 0.75 is 0.5 . The transform function must also pass throughthe points (0,0) and (1.5,1.5), meaning that FDS must compute the coefficients for the cubic polynomialf (ξ) = c0 +c1 ξ+c2 ξ2 +c3 ξ3. More constraints on the function lead to higher order polynomial functions,so be careful about too many constraints which could lead to non-monotonic functions. The monotonicityof the function is checked by the program and an error message is produced if it is not monotonic.

Do not specify either linear transformation points or IDERIV=0 points at coordinate values correspondingto the mesh boundaries.

6.3.6 Choosing Optimum Mesh Dimensions

A common question asked by new FDS users is, “What size mesh should I use?” The answer is not easybecause it depends considerably on what you are trying to accomplish. In general, you should build an FDSinput file using a relatively coarse mesh, and then gradually refine the mesh until you do not see appreciabledifferences in your results. Formally, this is referred to as a mesh sensitivity study.

For simulations involving buoyant plumes, a measure of how well the flow field is resolved is given bythe non-dimensional expression D∗/δx, where D∗ is a characteristic fire diameter

D∗ =(

Qρ∞ cp T∞

√g

) 25

(6.1)

and δx is the nominal size of a mesh cell1. The quantity D∗/δx can be thought of as the number of compu-tational cells spanning the characteristic (not necessarily the physical) diameter of the fire. The more cellsspanning the fire, the better the resolution of the calculation. It is better to assess the quality of the mesh interms of this non-dimensional parameter, rather than an absolute mesh cell size. For example, a cell size of10 cm may be “adequate,” in some sense, for evaluating the spread of smoke and heat through a buildingfrom a sizable fire, but may not be appropriate to study a very small, smoldering source2.

1The characteristic fire diameter is related to the characteristic fire size via the relation Q∗ = (D∗/D)5/2, where D is the physicaldiameter of the fire.

2For the validation study sponsored by the U.S. Nuclear Regulatory Commission [4], the D∗/δx values ranged from 4 to 16.

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6.4 Miscellaneous Parameters: The MISC Namelist Group (Table 15.12)

MISC is the namelist group of global miscellaneous input parameters. It contains parameters that do notlogically fit into any other category.

6.4.1 Basics

Only one MISC line should be entered in the data file. For example, the input line

&MISC SURF_DEFAULT='CONCRETE',TMPA=25. /

establishes that all bounding surfaces are to be made of CONCRETE unless otherwise specified, and that theambient temperature is 25 C.

The MISC parameters vary in scope and degree of importance. Here is a partial list of MISCellaneousparameters. Others are described where necessary throughout this guide.

DNS A logical parameter that, if .TRUE., directs FDS to perform a Direct Numerical Simulation, as opposedto the default Large Eddy Simulation (LES). This feature is appropriate only for simulations that usemesh cells that are on the order of a millimeter or less in size, or for diagnostic purposes.

GVEC The 3 components of gravity, in m/s2. The default is GVEC=0,0,-9.81.

HUMIDITY Relative humidity, in units of %. This need only be specified if there is a source of water in thesimulation other than the fire itself. Otherwise, water vapor is not explicitly tracked. Default 40 %.

ISOTHERMAL A logical parameter that indicates that the calculation does not involve any changes in tem-perature or radiation heat transfer, thus reducing the number of equations that must be solved, andsimplifying those that are. Automatically sets RADIATION to .FALSE.

NOISE FDS initializes the flow field with a very small amount of “noise” to prevent the development of aperfectly symmetric flow when the boundary and initial conditions are perfectly symmetric. To turn thisoff, set NOISE=.FALSE.

P_INF Background pressure (at the ground) in Pa. The default is 101325 Pa.

RADIATION A logical parameter indicating whether radiation transport ought to be calculated. The defaultis .TRUE.

SUPPRESSION A logical parameter indicating whether FDS should include gas phase flame extinction. Thedefault is .TRUE.

SURF_DEFAULT The SURF line that is to be applied to all boundaries, unless otherwise specified. Thedefault is ’INERT’, a non-reacting solid boundary whose temperature is fixed at TMPA. You do not needto define ’INERT’ via a SURF line.

TMPA Ambient temperature, the temperature of everything at the start of the simulation. The default is20 C.

U0, V0, W0 Initial values of the gas velocity in each of the coordinate directions. Normally, these are all0 m/s, but there are a few applications where it is convenient to start the flow immediately, like in anoutdoor simulation involving wind.

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6.4.2 Special Topic: Stopping and Restarting Calculations

An important MISC parameter is called RESTART. Normally, a simulation consists of a sequence of eventsstarting from ambient conditions. However, there are occasions when you might want to stop a calculation,make a few limited adjustments, and then restart the calculation from that point in time. To do this, firstbring the calculation to a halt gracefully by creating a file called CHID.stop in the directory where theoutput files are located. Remember that FDS is case-sensitive. The file name must be exactly the same asthe CHID and ‘stop’ should be lower case. FDS checks for the existence of this file at each time step, andif it finds it, gracefully shuts down the calculation after first creating a final Plot3D file and a file (or filesin the case of a multiple mesh job) called CHID.restart (or CHID_nn.restart). To restart a job, the file(s)CHID.restart should exist in the working directory, and the phrase RESTART=.TRUE. needs to be addedto the MISC line of the input data file. For example, suppose that the job whose CHID is “plume” is haltedby creating a dummy file called plume.stop in the directory where all the output files are being created. Torestart this job from where it left off, add RESTART=.TRUE. to the MISC line of the input file plume.fds, orwhatever you have chosen to name the input file. The existence of a restart file with the same CHID as theoriginal job tells FDS to continue saving the new data in the same files as the old. If RESTART_CHID is alsospecified on the MISC line, then FDS will look for old output files tagged with this string instead of usingthe specified CHID on the HEAD line. In this case, the new output files will be tagged with CHID, and the oldoutput files will not be altered.

When running the restarted job, the diagnostic output of the restarted job is appended to the file CHID.outthat was created by the original job. All of the other output files from the original run are appended as well.

There may be times when you want to save restart files periodically during a run as insurance againstpower outages or system crashes. If this is the case, at the start of the original run set DT_RESTART=50. onthe DUMP line to save restart files every 50 s, for example. The default for DT_RESTART is 1000000, meaningno restart files are created unless you gracefully stop a job by creating a dummy file called CHID.stop.

It is also possible to use the new control function feature (see Section 13.5) to stop a calculation or dumpa restart file when the computation reaches some measurable condition such as a first sprinkler activation.

Between job stops and restarts, major changes cannot be made in the calculation like adding or removingvents and obstructions. The changes are limited to those parameters that do not instantly alter the existingflow field. Since the restart capability has been used infrequently by the developers, it should be considereda fragile construct. Examine the output to ensure that no sudden or unexpected events occur during the stopand restart.

6.4.3 Special Topic: Defying Gravity

Most users of FDS assume that the acceleration of gravity points in the negative z direction, or more simply,downward. However, to change the direction of gravity to model a sloping roof or tunnel, for example,specify the gravity vector on the MISC line with a triplet of numbers of the form GVEC=0.,0.,-9.81, withunits of m/s2. This is the default, but it can be changed to be any direction.

There are a few special applications where you might want to vary the gravity vector as a function of timeor as a function of the first spatial coordinate, x. For example, on board the Space Shuttle or InternationalSpace Station, small motions can cause temporal changes in the normally zero level of gravity, an effectknown as “g-jitter.” More commonly, in tunnel fire simulations, it is sometimes convenient to change thedirection of gravity to mimic the change in slope. The slope of the tunnel might change as you travelthrough it; thus, you can tell FDS where to redirect gravity. For either a spatially or temporally varyingdirection and/or magnitude of gravity, do the following. First, on the MISC line, set the three components ofgravity, GVEC, to some “base” state like GVEC=1.,1.,1., which gives you the flexibility to vary all threecomponents. Next, designate “ramps” for the individual components, RAMP_GX, RAMP_GY, and RAMP_GZ,

37

all of which are specified on the MISC line. There is more discussion of RAMPs in Section 10, but for nowyou can use the following as a simple template to follow:

&MISC GVEC=1.,0.,1., RAMP_GX='x-ramp', RAMP_GZ='z-ramp' /

&RAMP ID='x-ramp', X= 0., F=0.0 /&RAMP ID='x-ramp', X= 50., F=0.0 /&RAMP ID='x-ramp', X= 51., F=-0.49 /&RAMP ID='x-ramp', X=100., F=-0.49 /

&RAMP ID='z-ramp', X= 0., F=-9.81 /&RAMP ID='z-ramp', X= 50., F=-9.81 /&RAMP ID='z-ramp', X= 51., F=-9.80 /&RAMP ID='z-ramp', X=100., F=-9.80 /

Note that both the x and z components of gravity are functions of x. FDS has been programmed to only allowvariation in the x coordinate. Note also that F is just a multiplier of the “base” gravity vector components,given by GVEC. This is why using the number 1 is convenient – it allows you to specify the gravity compo-nents on the RAMP lines directly. The effect of these lines is to model the first 50 m of a tunnel without aslope, but the second 50 m with a 5 % slope upwards. Note that the angle from vertical of the gravity vectordue to a 5 % slope is tan−1 0.05 = 2.86 and that 0.49 and 9.80 are equal to the magnitude of the gravityvector, 9.81 m/s2, multiplied by the sine and cosine of 2.86, respectively. To check your math, the sum ofthe squares of the gravity components ought to equal 9.81. Notice in this case that the y direction has beenleft out because there is no y variation in the gravity vector.

To vary the direction and/or magnitude of gravity in time, follow the same procedure but replace the Xin the RAMP lines with a T.

6.4.4 Special Topic: The Baroclinic Vorticity

The pressure term in the momentum transport equation solved by FDS is decomposed as follows:

1ρ

∇p = ∇

(pρ

)− p∇

(1ρ

)(6.2)

The pressure term is written like this so that a separable elliptic partial differential equation can be solvedfor the “total” pressure, H ≡ |u|2/2 + p/ρ, using a direct solver. The second term is calculated basedon the pressure field from the previous time step, a slight approximation necessary to render the pressureequation separable. This term is sometimes referred to as the baroclinic torque, and it is responsible forgenerating vorticity due to the non-alignment of pressure and density gradients. In versions of FDS prior to5.5, the second term was included only as an option. Starting with FDS 5.5, however, the baroclinic torque isincluded by default. It can be excluded by setting BAROCLINIC=.FALSE. on the MISC line, but this is onlyrecommended for diagnostic purposes. For example, in the simple helium plume test case below, neglectingthe baroclinic torque changes the puffing behavior noticeably. In other applications, however, its effect isless significant. For further discussion of its effect, see Ref. [5].

Example Case: helium_2d

This case demonstrates the use of baroclinic correction for an axially-symmetric helium plume. Note that thegoverning equations solved in FDS are written in terms of a three dimensional Cartesian coordinate system.However, a two dimensional Cartesian or two dimensional cylindrical (axially-symmetric) calculation canbe performed by setting the number of cells in the y direction to 1. An example of an axially-symmetrichelium plume is shown in Figure 6.5.

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&HEAD CHID='helium_2d',TITLE='Axisymmetric Helium Plume' /&MESH IJK=72,1,144 XB=0.00,0.08,-0.001,0.001,0.00,0.16, CYLINDRICAL=.TRUE. /&TIME TWFIN=5.0 /&MISC DNS=.TRUE., ISOTHERMAL=.TRUE. /&SPEC ID='HELIUM' /&SURF ID='HELIUM', VEL=-0.673, MASS_FRACTION(1)=1.0, TAU_MF(1)=0.3 /&VENT MB='XMAX' ,SURF_ID='OPEN' /&VENT MB='ZMAX' ,SURF_ID='OPEN' /&OBST XB= 0.0,0.036,-0.001,0.001,0.00,0.02, SURF_IDS='HELIUM','INERT','INERT' /&DUMP PLOT3D_QUANTITY(1)='PRESSURE',PLOT3D_QUANTITY(5)='HELIUM' /&SLCF PBY=0.000,QUANTITY='DENSITY', VECTOR=.TRUE. /&SLCF PBY=0.000,QUANTITY='HELIUM' /&TAIL /

Figure 6.5: Simulation of a helium plume.

6.4.5 Special Topic: Stack Effect

Tall buildings often experience buoyancy-induced air movement due to temperature differences betweeninside and outside, known as stack effect. To simulate this phenomenon in FDS, you must include the entirebuilding, or a substantial fraction of it, both inside and out, in the computational domain. It is important tocapture the pressure and density decrease in the atmosphere based on the specified temperature LAPSE_RATE(C/m) that is entered on the MISC line. Experiment with different meshing strategies before including anyfire or HVAC functionality. Slowly build in complexity.

Example Case: stack_effect

If the interior temperature of a building is at a different temperature than the surrounding atmosphere, up-ward or downward air flows within shafts or stairwells connected to the ambient via leakage paths will occur.This phenomena is known as the stack effect. The stack_effect test case is a 2D simulation of a 304 m tallbuilding initialized to a temperature of 20 C with the surrounding ambient temperature initialized to 10 C.Two small openings in the building are defined 2.5 m above the ground floor of the building and 2.5 m belowthe roof of the building. The initial density stratification is defined by assuming a lapse rate of 0 C/m.

ρ0(z) = ρ∞ egW

R0T0z (6.3)

Applying this to the external and internal locations at the lower and upper vents results in densities of1.2392, 1.1969, 1.1954, and 1.1546 kg/m3, respectively. FDS computes the same values to within machineprecision. Since the openings in the building are equally spaced over its height, the neutral plane of thebuilding will be close to its midpoint. The pressure gradient across the building’s wall can be computed as

δP =WP0g

R0

(1

Tambient− 1

Tbuilding

)h (6.4)

where h is the distance from the neutral plane. Using this pressure gradient in Bernoulli’s equation (andassuming it remains constant) results in a velocity of 10.09 m/s through the vent. FDS computes a peakvelocity of 10.13 m/s, an error of 0.5 %.

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6.4.6 Special Topic: Large Eddy Simulation Parameters

In default mode, FDS uses the Smagorinsky form of Large Eddy Simulation (LES) to model subgrid-scaleturbulence. The viscosity µ is modeled

µLES = ρ(Cs ∆)2(

2 Si j : Si j−23(∇ ·u)2

) 12

(6.5)

where Cs is an empirical constant and ∆ is a length on the order of the size of a grid cell. The bar abovethe various quantities denotes that these are the resolved, or filtered, values, meaning that they are computedon a numerical grid. The other diffusive parameters, the thermal conductivity and material diffusivity, arerelated to the turbulent viscosity by

kLES =µLES cp

Prt; (ρD)l,LES =

µLES

Sct(6.6)

The turbulent Prandtl number Prt and the turbulent Schmidt number Sct are assumed to be constant for agiven scenario. Although it is not recommended for most calculations, you can modify Cs = 0.2, Prt = 0.5,and Sct = 0.5 via the parameters CSMAG, PR, and SC on the MISC line. A more detailed discussion of theseparameters is given in the FDS Technical Reference Guide [6].

6.4.7 Special Topic: Numerical Stability Parameters

The time step of an FDS simulation is constrained by the convective and diffusive transport speeds via twoconditions. The first is known as the Courant-Friedrichs-Lewy (CFL) condition. The CFL condition assertsthat the solution of the equations cannot be updated with a time step larger than that which would allow aparcel of fluid to travel further than a single mesh cell. In each mesh cell of dimension δx by δy by δz withvelocity components u, v, and w, the CFL number is defined:

CFL = δt max( |u|

δx,|v|δy

,|w|δz

)(6.7)

Every time step, the CFL number is computed in each mesh cell, and the time step, δt, is adjusted if themaximum value of the CFL number is not between CFL_MIN and CFL_MAX, whose default values are 0.8and 1.0, respectively. These values are included in the MISC namelist group.

A similar condition, but one constraining the time step when diffusive transport dominates, is sometimescalled the Von Neumann condition. The Von Neumann number is defined:

VN = 2 max(

ν,D,k

ρcp

)δt(

1δx2 +

1δy2 +

1δz2

)(6.8)

Like the CFL number, VN is computed in each mesh cell, and the time step is adjusted if VN is outside therange between VN_MIN and VN_MAX, which are 0.8 and 1.0 by default. Note that this constraint is appliedto the momentum, mass and energy equations via the relevant diffusion parameter – viscosity, materialdiffusivity or thermal conductivity. This constraint on the time step is typical of any explicit, second-ordernumerical scheme for solving a parabolic partial differential equation. To save CPU time, the Von Neumanncriterion is only invoked for DNS calculations or for LES calculations with mesh cells smaller than 5 mm.

Resetting the stability parameters is not recommended except in very special circumstances, as they can leadto simulations failing due to numerical instabilities.

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If you want to prevent FDS from automatically changing the time step, set LOCK_TIME_STEP=.TRUE. onthe TIME line, in which case the specified time step, DT, will not be adjusted. This parameter is intended fordiagnostic purposes only, for example, timing program execution. It can lead to numerical instabilities if theinitial time step is set too high.

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6.5 Special Topic: Unusual Initial Conditions: The INIT Namelist Group(Table 15.8)

Usually, an FDS simulation begins at time t = 0 with ambient conditions. The air temperature is assumedconstant with height, and the density and pressure decrease with height (the z direction). This decrease isnot noticed in most building scale calculations, but it is important in large outdoor simulations. There aresome scenarios for which it is convenient to change the ambient conditions within some rectangular regionof the domain. If so, add lines of the form

&INIT XB=0.5,0.8,2.1,3.4,2.5,3.6, TEMPERATURE=30. /

Here, within the region whose bounds are given by the sextuplet XB, the initial temperature shall be 30 Cinstead of the ambient. This construct can also be used for DENSITY or MASS_FRACTION(N) where N

indicates the Nth species listed in the input file. Make sure that you specify all species (components ofMASS_FRACTION(N)) on the same INIT line.

The INIT construct may be useful in examining the influence of stack effect in a building, where thetemperature is different inside and out.

Note that a solid obstruction can be given an initial temperature via the parameter TMP_INNER on theSURF line. An initial velocity can be prescribed via U0, V0, and W0 on the MISC line.

6.6 Special Topic: Improving the Pressure Solver: The PRESNamelist Group(Table 15.16)

FDS uses a low-Mach number formulation of the Navier-Stokes equations. One of the consequences of thisis that the speed of sound is assumed infinite, and that the pressure throughout the computational domain isaffected, instantaneously, by local changes in the flow field. A simple example of this is when air is pushedthrough a tunnel. If the tunnel has forced flow at one end and an opening at the other, the volume flow at theopening is the same as that which is forced at the other end. Without any heat addition, the air is assumedincompressible. Information is passed through the tunnel instantaneously in the model via a solution of alinear system of equations for the pressure. For a single mesh, the solution of this Poisson equation forthe pressure is very accurate. However, for multiple meshes, there is potentially a delay in informationpassing throughout the domain because the Poisson equation is solved on each individual mesh, withoutany influence from the larger computational domain. The details of the numerics can be found in the FDSTechnical Reference Guide.

Another limitation of the pressure solver is that at solid surfaces that are not part of the boundary of thecomputational domain, the pressure solver enforces a no-flux boundary condition. However, it is not perfect,and it is possible to have a non-zero normal velocity at a solid surface. For most applications, this velocityis so small that it has a negligible effect on the solution.

If either the error in the normal component of the velocity at a mesh interface or at a solid boundaryis large, you can reduce it by making more than the default number of calls to the pressure solver at eachtime step. To do so, specify VELOCITY_TOLERANCE on the PRES line to be the maximum allowable normalvelocity component on the solid boundary or the largest error at a mesh interface. It is in units of m/s. If youset this, experiment with different values, and monitor the number of pressure iterations required at eachtime step to achieve your desired tolerance. The number of iterations are written out to the file CHID.out.If you use a value that is too small, the CPU time required might be prohibitive. The maximum number ofiterations per time step is given by MAX_PRESSURE_ITERATIONS, also on the PRES line. Its default valueis 10000.

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Example Case: duct_flow

To demonstrate how to improve the accuracy of the pressure solver, consider the flow of air through a squareduct that crosses several meshes. In the case called duct_flow, air is pushed through a 1 m2 duct at 1 m/s.With no thermal expansion, the volume flow into the duct ought to equal the volume flow out of the duct.Figure 6.6 displays the computed inflow and outflow as a function of time, and the number of pressureiterations required. The outflow does not match the inflow exactly because of inaccuracies at the solid andmesh boundaries. The VELOCITY_TOLERANCE has been set to 0.01 m/s.

0 10 20 30 40 50 600

0.5

1

1.5

Time (s)

Vol

ume

Flo

w(m

3/s

)

Volume Flow (duct flow)

0 10 20 30 40 50 600

20

40

60

80

100

Time (s)

Iter

atio

ns

Pressure Iterations (duct flow)

Figure 6.6: (Left) Volume flow into and out of a square duct. (Right) The number of pressure iterations as afunction of time.

6.7 Special Topic: Setting Limits: The CLIP Namelist Group (Table 15.2)

On rare occasions you might need to set upper or lower bounds on the density, temperature, or species massfractions. The parameters listed in Table 15.2 are for diagnostic purposes only.

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44

Chapter 7

Building the Model

A considerable amount of work in setting up a calculation lies in specifying the geometry of the spaceto be modeled and applying boundary conditions to these objects. The geometry is described in terms ofrectangular obstructions that can heat up, burn, conduct heat, etc.; and vents from which air or fuel can beeither injected into, or drawn from, the flow domain. A boundary condition needs to be assigned to eachobstruction and vent describing its thermal properties. A fire is just one type of boundary condition. Thischapter describes how to build the model.

7.1 Bounding Surfaces: The SURF Namelist Group (Table 15.24)

Before describing how to build up the geometry, it is first necessary to explain how to describe what thesebounding surfaces consist of. SURF is the namelist group that defines the structure of all solid surfaces oropenings within or bounding the flow domain. Boundary conditions for obstructions and vents are prescribedby referencing the appropriate SURF line(s) whose parameters are described in this section.

The default boundary condition for all solid surfaces is that of a inert wall with the temperature fixed atTMPA, and is referred to as ’INERT’. If only this boundary condition is needed, there is no need to add anySURF lines to the input file. If additional boundary conditions are desired, they are to be listed one boundarycondition at a time. Each SURF line consists of an identification string ID=’...’ to allow references to it byan obstruction or vent. Thus, on each OBST and VENT line that are to be described below, the character stringSURF_ID=’...’ indicates the ID of the SURF line containing the desired boundary condition parameters.If a particular SURF line is to be applied as the default boundary condition, CONCRETE for example, setSURF_DEFAULT=’CONCRETE’ on the MISC line.

7.2 Creating Obstructions: The OBST Namelist Group (Table 15.14)

The namelist group OBST contains parameters used to define obstructions. The entire geometry of the modelis made up entirely of rectangular solids, each one introduced on a single line in the input file.

7.2.1 Basics

Each OBST line contains the coordinates of a rectangular solid within the flow domain. This solid is definedby two points (x1,y1,z1) and (x2,y2,z2) that are entered on the OBST line in terms of the sextupletXB = X1, X2, Y1, Y2, Z1, Z2

In addition to the coordinates, the boundary conditions for the obstruction can be specified with the param-eter SURF_ID, which designates which SURF group (Section 7.1) to apply at the surface of the obstruction.

45

If the obstruction has different properties for its top, sides and bottom, do not specify only one SURF_ID.Instead, use SURF_IDS, an array of three character strings specifying the boundary condition IDs for thetop, sides and bottom of the obstruction, respectively. If the default boundary condition is desired, thenSURF_ID(S) need not be set. However, if at least one of the surface conditions for an obstruction is theinert default, it can be referred to as ’INERT’, but it does not have to be explicitly defined. For example:

&SURF ID='FIRE',HRRPUA=1000.0 /&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_IDS='FIRE','INERT','INERT' /

puts a fire on top of the obstruction. This is a simple way of prescribing a burner.

Some additional features of obstructions are as follows:

• In addition to SURF_ID and SURF_IDS, you can also use the sextuplet SURF_ID6 as follows:

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_ID6='FIRE','INERT','HOT','COLD','BLOW','INERT' /

where the six surface descriptors refer to the planes x = 2.3, x = 4.5, y = 1.3, y = 4.8, z = 0.0, andz = 9.2, respectively. Note that SURF_ID6 should not be used on the same OBST line as SURF_ID orSURF_IDS.

• Obstructions can have zero thickness. Often, thin sheets, like a window, form a barrier, but if thenumerical mesh is coarse relative to the thickness of the barrier, the obstruction might be unnecessarilylarge if it is assumed to be one layer of mesh cells thick. All faces of an obstruction are shifted to theclosest mesh cell. If the obstruction is very thin, the two faces may be approximated on the same cellface. FDS and Smokeview render this obstruction as a thin sheet, but it is allowed to have thermallythick boundary conditions. This feature is fragile, especially in terms of burning and blowing gas. Athin sheet obstruction can only have one velocity vector on its face, thus a gas cannot be injected reliablyfrom a thin obstruction because whatever is pushed from one side is necessarily pulled from the other.For full functionality, the obstruction should be specified to be at least one mesh cell thick. Thin sheetobstructions work fine as flow barriers, but other features are fragile and should be used with caution.To prevent FDS from allowing thin sheet obstructions, set THICKEN_OBSTRUCTIONS=.TRUE. on theMISC line, or THICKEN=.TRUE. on each OBST line for which the thin sheet assumption is not allowed.

• Unlike earlier versions of FDS, obstructions that are too small relative to the underlying numerical meshare rejected. Be careful when testing cases on coarse meshes.

• Obstructions may be created or removed during a simulation. See Section 13.4.1 for details.

• If two obstructions overlap at one or more faces, the one listed last in the input file takes precedence overthe one listed first, in the sense that the latter’s surface properties will be applied to the overlapping face.Smokeview renders both obstructions independently of each other, often leading to an unsightly cross-hatching of the two surface colors where there is an overlap. A simple remedy for this is to “shrink” thefirst obstruction slightly by adjusting its coordinates (XB) accordingly. Then, in Smokeview, toggle the“q” key to show the obstructions as you specified them, rather than as FDS rendered them.

• Obstructions can be protected from the HOLE punching feature. Sometimes it is convenient to createa door or window using a HOLE. For example, suppose a HOLE is punched in a wall to represent adoor or window. An obstruction can be defined to fill this hole (presumably to be removed or col-ored differently or whatever) so long as the phrase PERMIT_HOLE=.FALSE. is included on the OBST

46

line. In general, any OBSTruction can be made impenetrable to a HOLE using this phrase. By de-fault, PERMIT_HOLE=.TRUE., meaning that an OBSTruction is assumed to be penetrable unless oth-erwise directed. Note that if an penetrable OBSTruction and an inpentetrable OBSTruction overlap, theOBSTruction with PERMIT_HOLE=.FALSE. should be listed first.

• If the obstruction is not to be removed or rejected for any reason, set REMOVABLE=.FALSE. This issometimes needed to stop FDS from removing the obstruction if it is embedded within another, like adoor within a wall.

• In rare cases, you might not want to allow a VENT to be attached to a particular obstruction, in whichcase set ALLOW_VENT=.FALSE.

• Obstructions can be made semi-transparent by assigning a TRANSPARENCY on the OBST line. This realparameter ranges from 0 to 1, with 0 being fully transparent. The parameter should always be set alongwith either COLOR or an RGB triplet. It can also be specified on the appropriate SURF line, along with acolor indicator.

• Obstructions are drawn solid in Smokeview. To draw an outline representation, set OUTLINE=.TRUE.

7.2.2 Repeated Obstructions: The MULT Namelist Group (Table 15.13)

Sometimes obstructions are repeated over and over in the input file. This can be tedious to create and makethe input file hard to read. However, if a particular obstruction or set of obstructions repeats itself in a regularpattern, you can use a utility known as a multiplier. If you want to repeat an obstruction, create a line in theinput file as follows:

&MULT ID='m1', DX=1.2, DY=2.4, I_LOWER=-2, I_UPPER=3, J_LOWER=0, J_UPPER=5 /

&OBST XB=..., MULT_ID='m1' /

This has the effect of making an array of obstructions according to the following formulae:

xi = x0 +δx0 + iδx ; I_LOWER≤ i≤ I_UPPER

y j = y0 +δy0 + j δy ; J_LOWER≤ j ≤ J_UPPER

Note that the same rules apply for the z direction as well. In situations where the position of the obstructionneeds shifting prior to the multiplication, use the parameters DX0, DY0, and DZ0.

A generalization of this idea is to replace the parameters, DX, DY, and DZ, with a sextuplet called DXB.The six entries in DXB increment the respective values of the obstruction coordinates given by XB. Forexample, define the lower y bound of the original obstruction by XB(3,0). The nth obstruction would be:

XB(3,N) = XB(3,0) + N*DXB(3)

Notice that we use N_LOWER and N_UPPER to denote the range of N. This more flexible input scheme allowsyou to create, for example, a slanted roof in which the individual roof segments shorten as they ascend tothe top. This feature is demonstrated by the following short input file that creates a hollowed out pyramidusing the four perimeter obstructions that form the outline of its base:

&HEAD CHID='pyramid', TITLE='Simple demo of multiplier function' /&MESH IJK=100,100,100, XB=0.0,1.0,0.0,1.0,0.0,1.0 /&TIME T_END=0. /

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Figure 7.1: An example of the multiplier function.

&MULT ID='south', DXB=0.01,-.01,0.01,0.01,0.01,0.01, N_LOWER=0, N_UPPER=39 /&MULT ID='north', DXB=0.01,-.01,-.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 /&MULT ID='east', DXB=-.01,-.01,0.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 /&MULT ID='west', DXB=0.01,0.01,0.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 /&OBST XB=0.10,0.90,0.10,0.11,0.10,0.11, MULT_ID='south', COLOR='RED' /&OBST XB=0.10,0.90,0.89,0.90,0.10,0.11, MULT_ID='north', COLOR='BLUE' /&OBST XB=0.10,0.11,0.11,0.89,0.10,0.11, MULT_ID='west', COLOR='GREEN' /&OBST XB=0.89,0.90,0.11,0.89,0.10,0.11, MULT_ID='east', COLOR='CYAN' /

The end result of this input file is to create a pyramid by repeating long, rectangular obstructions at the baseof each face in a stair-step pattern. Note in this case the use of N_LOWER and N_UPPER which automaticallycause FDS to repeat the obstructions in sequence rather than as an array.

Note that the MULTiplication functionality works for MESHes, as well, but it only applies to the bounds(XB) of the mesh, not the number of cells.

7.2.3 Non-rectangular Geometry and Sloped Ceilings

The efficiency of FDS is due to the simplicity of its numerical mesh. However, there are situations in whichcertain geometric features do not conform to the rectangular mesh, such as a sloped ceiling or roof. In thesecases, construct the curved geometry using rectangular obstructions, a process sometimes called “stair-stepping”. A concern is that the stair-stepping changes the flow pattern near the wall. To lessen the impactof stair-stepping on the flow field near the wall, prescribe the parameter SAWTOOTH=.FALSE. on each OBSTline that makes up the stair-stepped obstruction. The effect of this parameter is to prevent vorticity frombeing generated at sharp corners, in effect smoothing out the jagged steps that make up the obstruction. Thisis not a complete solution of the problem, but it does provide a simple way of ensuring that the flow fieldaround a non-rectangular obstruction is not inhibited by extra drag created at sharp corners.

Do not apply SAWTOOTH=.FALSE. to obstructions that have any SURF_IDs with the attributeBURN_AWAY=.TRUE.

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Example Case: sawtooth

In this example, we look at the flow field past a diagonally oriented obstruction. If SAWTOOTH=.FALSE.,then the velocity boundary conditions will be applied in such a way as to minimize the impact of the bound-aries due to vortices at sharp corners, as shown in the following example:

&OBST XB= 0.00, 0.05,-0.01, 0.01, 0.00, 0.05, SAWTOOTH=.FALSE., COLOR='EMERALD GREEN' /&OBST XB= 0.05, 0.10,-0.01, 0.01, 0.00, 0.10, SAWTOOTH=.FALSE., COLOR='EMERALD GREEN' /&OBST XB= 0.10, 0.15,-0.01, 0.01, 0.05, 0.15, SAWTOOTH=.FALSE., COLOR='EMERALD GREEN' /&OBST XB= 0.15, 0.20,-0.01, 0.01, 0.10, 0.20, SAWTOOTH=.FALSE., COLOR='EMERALD GREEN' /

In Figure 7.2, the top set of obstructions are using the default SAWTOOTH=.TRUE. and the bottom set ofobstructions are using SAWTOOTH=.FALSE. The adjacent obstructions that have SAWTOOTH=.FALSE. aredisplayed in Smokeview as one smooth obstruction, shown in green. Notice that as the air moves across thedifferent sets of obstructions, the air velocity on the bottom set of obstructions is not affected as much bythe vortices.

Figure 7.2: Simple example of SAWTOOTH=.FALSE.

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7.3 Creating Voids: The HOLE Namelist Group (Table 15.7)

The HOLE namelist group is used to define parameters (Table 15.7) to carve a hole out of an existing ob-struction or set of obstructions. To do this, add lines of the form

&HOLE XB=2.0,4.5,1.9,4.8,0.0,9.2 /

Any solid mesh cells within the volume 2.0 < x < 4.5, 1.9 < y < 4.8, 0.0 < z < 9.2 are removed. Obstruc-tions intersecting the volume are broken up into smaller blocks.

If the hole represents a door or window, a good rule of thumb is to punch more than enough to create thehole. This ensures that the hole is created through the entire obstruction.

For example, if the OBST line denotes a wall 0.1 m thick:

&OBST XB=1.0,1.1,0.0,5.0,0.0,3.0 /

and you want to create a door, add this:

&HOLE XB=0.99,1.11,2.0,3.0,0.0,2.0 /

The extra centimeter added to the x coordinates of the hole make it clear that the hole is to punch throughthe entire obstruction.

When a HOLE is created, the affected obstruction(s) are either rejected, or created or removed at pre-determined times. See Section 13.4.1 for details. To allow a hole to be controlled with either the CTRL orDEVC namelist groups, you will need to add the CTRL_ID or DEVC_ID parameter respectively, to the HOLEline.

If you want the obstruction that is to be cut out to have a different color than the original obstruction,set the COLOR or integer triplet RGB on the HOLE line (see Section 7.5).

When a HOLE is in a .FALSE. state, an obstruction is placed in the hole. To make this obstruc-tion transparent, the TRANSPARENCY parameter should be specified by a real number from 0 to 1. Notethat if TRANSPARENCY is specified, then either a COLOR or RGB triplet ought to be specified as well. ATRANSPARENCY value near, but not equal to, zero can be used to simulate a window when the HOLE’sINITIAL_STATE=.FALSE. When the DEVC or CTRL is activated and changes the state of the hole to.TRUE., the HOLE is then open and completely transparent. See Section 13.4.1 for an example.

If an obstruction is not to be punctured by a HOLE, add PERMIT_HOLE=.FALSE. to the OBST line.

It is a good idea to inspect the geometry by running either a setup job (T_END=0 on the TIME line) or ashort-time job to test the operation of devices and control functions.

Note that a HOLE has no effect on a VENT or a mesh boundary. It only applies to OBSTstructions.

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7.4 Applying Surface Properties: The VENT Namelist Group (Table 15.28)

Whereas the OBST group is used to specify obstructions within the computational domain, the VENT group(Table 15.28) is used to prescribe planes adjacent to obstructions or external walls. Note that the label VENTis used for historical reasons – this group of parameters has evolved well beyond its initial role as simplyallowing for air to be blown into, or sucked out of, the computational domain.

7.4.1 Basics

The vents are chosen in a similar manner to the obstructions, with the sextuplet XB denoting a plane abuttinga solid surface. Two of the six coordinates must be the same, denoting a plane as opposed to a solid.

Note that only one VENT may be specified for any given wall cell. If additional VENT lines are specified fora given wall cell, FDS will output a warning message and ignore the subsequent lines (i.e. only the first ventwill be applied)

The term “VENT” is somewhat misleading. Taken literally, a VENT can be used to model components ofthe ventilation system in a building, like a diffuser or a return. In these cases, the VENT coordinates form aplane on a solid surface forming the boundary of the duct. No holes need to be created through the solid; itis assumed that air is pushed out of or sucked into duct work within the wall. Less literally, a VENT is usedsimply as a means of applying a particular boundary condition to a rectangular patch on a solid surface.A fire, for example, is usually created by first generating a solid obstruction via an OBST line, and thenspecifying a VENT somewhere on one of the faces of the solid with a SURF_ID with the characteristics ofthe thermal and combustion properties of the fuel. For example, the lines

&OBST XB=0.0,5.0,2.0,3.0,0.0,4.0, SURF_ID='big block' /&VENT XB=1.0,2.0,2.0,2.0,1.0,3.0, SURF_ID='hot patch' /

specify a large obstruction (with the properties given elsewhere in the file under the name ’big block’)with a “patch” applied to one of its faces with alternative properties under the name ’hot patch’. Thislatter surface property need not actually be a “vent,” like a supply or return duct, but rather just a patch withdifferent boundary conditions than those assumed for the obstruction. Note that the surface properties of aVENT over-ride those of the underlying obstruction.

Unlike previous versions of FDS, you can no longer specify a free-standing fan using the VENT construct. AVENT must always be attached to a solid obstruction. See Section 9.1 for instructions on specifying differenttypes of fans.

An easy way to specify an entire external wall is to replace XB with MB (Mesh Boundary), a character stringwhose value is one of the following: ’XMAX’, ’XMIN’, ’YMAX’, ’YMIN’, ’ZMAX’ or ’ZMIN’ denoting theplanes x = XMAX, x = XMIN, y = YMAX, y = YMIN, z = ZMAX or z = ZMIN, respectively. Like an obstruction,the boundary condition index of a vent is specified with SURF_ID, indicating which of the listed SURF linesto apply. If the default boundary condition is desired, then SURF_ID need not be set.

Be careful when using the MB shortcut when doing a multiple mesh simulation, that is, when morethan one rectangular mesh is used. The plane designated by the keyword MB is applied to all of the meshes,possibly leading to confusion about whether a plane is a solid wall or an open boundary. Check the geometryin Smokeview to assure that the VENTs are properly prescribed. Use color as much as possible to double-check the set-up. More detail on color in Section 7.5 and Table 7.1. Also, the parameter OUTLINE=.TRUE.causes the VENT to be drawn as an outline in Smokeview.

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7.4.2 Special VENTs

There are two reserved SURF_ID’s that may be applied to a VENT – ’OPEN’ and ’MIRROR’. The termreserved means that these two SURF_IDs should not be explicitly defined by you. Their properties arepredefined.

An OPEN VENT

The first special VENT is invoked by the parameter SURF_ID=’OPEN’. This is used only if the VENT isapplied to the exterior boundary of the computational domain, where it denotes a passive opening to theoutside. By default, FDS assumes that the exterior boundary of the computational domain (the XBs on theMESH line) is a solid wall. To change this, use an OPEN vent as if it were an open door or window. To createa totally or partially open domain, use OPEN vents on the exterior mesh boundaries (MBs).

By default, it is assumed that ambient conditions exist beyond the ’OPEN’ vent. However, in somecases, you may want to alter this assumption, for example, the temperature. If you assume a temperatureother than ambient, specify TMP_EXTERIOR along with SURF_ID=’OPEN’. Use this option cautiously – inmany situations if you want to describe the exterior of a building, it is better to include the exterior explicitlyin your calculation because the flow in and out of the doors and windows will be more naturally captured.See Section 6.4.5 for more details.

As with exterior temperature, to change the exterior mass fraction of a particular gas species, setMASS_FRACTION(N) on the VENT line, where N denotes the species index. See Section 11.2 for moreinformation about gas species.

If you want to specify a non-ambient pressure at the OPEN boundary, see Section 9.3.

Starting with FDS 5.3.0, vents to the outside of the computational domain (OPEN vents) can be opened orclosed during a simulation. It is best done by creating or removing a thin obstruction that covers the OPENVENT. See Section 13.4.2 for details.

A MIRROR VENT

A VENT with SURF_ID=’MIRROR’ denotes a symmetry plane. Usually, a MIRROR spans an entire face ofthe computational domain, essentially doubling the size of the domain with the MIRROR acting as a plane ofsymmetry. The flow on the opposite side of the MIRROR is exactly reversed. From a numerical point of view,a MIRROR is a no-flux, free-slip boundary. As with OPEN, a MIRROR can only be prescribed at an exteriorboundary of the computational domain. Often, OPEN or MIRROR VENTs are prescribed along an entire sideof the computational domain, in which case the “MB” notation is handy.

Note that the mirror image of a scene is not shown in Smokeview.

A word of warning about MIRROR boundaries in FDS. In conventional RANS (Reynolds-Averaged Navier-Stokes) models, symmetry boundaries are often used as a way of saving on computation time. However,because FDS is an LES (Large Eddy Simulation) model, the use of symmetry boundaries should be con-sidered carefully. The reason for this is that an LES model does not compute a time-averaged solution ofthe N-S equations. In other words, for a RANS model, a fire plume is represented as an axially-symmetricflow field because that is what you would expect if you time-averaged the actual flow field over a sufficientamount of time. Thus, for a RANS model, a symmetry boundary along the plume centerline is appropriate.In an LES model, however, there is no time-averaging built into the equations, and there is no time-averaged,

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symmetric solution. Putting a MIRROR boundary along the centerline of a fire plume will change its dynam-ics entirely. It will produce something very much like the flow field of a fire that is adjacent to a verticalwall. For this reason, a MIRROR boundary condition is not recommended along the centerline of a turbulentfire plume. If the fire or burner is very small, and the flow is laminar, then the MIRROR boundary conditionmakes sense. In fact, in 2-D calculations, MIRROR boundary conditions are employed in the third coordinatedirection (this is done automatically, you need not specify it explicitly).

7.4.3 Controlling VENTs

VENT functionality can be controlled in some cases using “devices” and “controls,” specified via a DEVC_IDor a CTRL_ID. See Section 13.4.2 for details.

7.4.4 Trouble-Shooting VENTs

Unlike most of the entries in the input file, the order that you specify VENTs can be important. There mightbe situations where it is convenient to position one VENT atop another. For example, suppose you want todesignate the ceiling of a compartment to have a particular set of surface properties, and you designate theentire ceiling to have the appropriate SURF_ID. Then, you want to designate a smaller patch on the ceilingto have another set of surface properties, like an air supply. In this case, you must designate the supplyVENT first because for that area of the ceiling, FDS will ignore the ceiling properties and apply the supplyproperties. FDS processes the first VENT, not the second as it did in versions prior to FDS 5. Now, the rulefor VENTs is “first come, first served.” Keep in mind, however, that the second VENT is not rejected entirely– only where there is overlap. FDS will also print out a warning to the screen (or to standard error) sayingwhich VENT has priority.

Smokeview can help identify where two VENTs overlap, assuming each has a unique COLOR. BecauseSmokeview draws VENTs on top of each other, areas of overlap will have a grainy, awkward appearancethat changes pattern as you move the scene. In situations where you desire the overlap for the sake ofconvenience, you might want to slightly adjust the coordinates of the preferred VENT so that it is slightlyoffset from the solid surface. Make the offset less than about a tenth of a cell dimension so that FDS snapsit to its desired location. Then, by toggling the “q” key in Smokeview, you can eliminate the grainy coloroverlap by showing the VENT exactly where you specified it, as opposed to where FDS repositioned it. Thistrick also works where the faces of two obstructions overlap.

If an error message appears requesting that the orientation of a vent be specified, first check to make surethat the vent is a plane. If the vent is a plane, then the orientation can be forced by specifying the parameterIOR. If the normal direction of the VENT is in the positive x direction, set IOR=1. If the normal directionis in the negative x direction, set IOR=-1. For the y and z direction, use the number 2 and 3, respectively.Setting IOR may sometimes solve the problem, but it is more likely that if there is an error message aboutorientation, then the VENT is buried within a solid obstruction, in which case the program cannot determinethe direction in which the VENT is facing.

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7.5 Coloring Obstructions, Vents, Surfaces and Meshes

Colors for many items within FDS can be prescribed in two ways; a triplet of integers after keyword RGB orone of many COLOR name character strings.

The three RGB integer numbers range from 0 to 255, indicating the amount of Red, Green and Blue thatmake up the color. If you define the COLOR by name, it is important that you type the name EXACTLY as itis listed in the color tables here in this document and on the FDS website.

Table 7.1 provides a small sampling of RGB values and COLOR names for a variety of colors. A completelisting of all 500+ colors that can be specified by name after the COLOR keyword is available on the FDSwebsite. If the COLOR name is not listed in the table on the website, then that name does not exist to FDS.

It is highly recommended that colors be assigned to surfaces via the SURF line because as the geometriesof FDS simulations become more complex, it is very useful to use color as a spot check to determine if thedesired surface properties have been assigned throughout the room or building under study.

For example, if you desire that all surfaces associated with a given SURF line be colored the same way,prescribe a triplet of integers called RGB on the SURF line. The following SURF line;

&SURF ID='UPHOLSTERY',...,RGB=0,255,0 /

will cause the furnishings with a “SURF” of “UPHOLSTERY” to be colored green in Smokeview. It is bestto avoid using the primary colors because these same colors are used by Smokeview to draw color contours.

Obstructions and vents may be colored individually (over-riding the SURF line’s RGB) by specifyingCOLOR value to any of the listed names in Table 7.1 or ’INVISIBLE’ on the respective OBST or VENT line.Using ’INVISIBLE’ causes the vent or obstruction to not be drawn.

Colors may also be specified using the integer triplet RGB on an OBST or VENT line to gain a wider colorpalette. The use of RGB is preferable, especially to create colors that do not clash with the pastel colors usedto show temperatures, concentrations, etc. See Table 7.1 for a list of color names and RGB values.

7.5.1 Texture Mapping

There are various ways of prescribing the color of various objects within the computational domain, butthere is also a way of pasting images onto the obstructions for the purpose of making the Smokeview imagesmore realistic. This technique is known as “texture mapping.” For example, to apply a wood paneling imageto a wall, add to the SURF line defining the physical properties of the paneling the text

&SURF ID='wood paneling',...,TEXTURE_MAP='paneling.jpg',TEXTURE_WIDTH=1.,TEXTURE_HEIGHT=2. /

Assuming that a JPEG file called paneling.jpg exists in the working directory, Smokeview should read itand display the image wherever the paneling is used (SGI Users: use rgb files instead of jpg). Note that theimage does not appear when Smokeview is first invoked. It is an option controlled by the Show/Hide menu.The parameters TEXTURE_WIDTH and TEXTURE_HEIGHT are the physical dimensions of the image. In thiscase, the JPEG image is of a 1 m wide by 2 m high piece of paneling. Smokeview replicates the image asoften as necessary to make it appear that the paneling is applied where desired. Consider carefully how theimage repeats itself when applied in a scene. If the image has no obvious pattern, there is no problem withthe image being repeated. If the image has an obvious direction, the real triplet TEXTURE_ORIGIN shouldbe added to the VENT or OBST line to which a texture map should be applied. For example,

&OBST XB=1.0,2.0,3.0,4.0,5.0,7.0,SURF_ID='wood paneling',TEXTURE_ORIGIN=1.0,3.0,5.0 /

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Table 7.1: Sample of Color Definitions (A complete list is included on the website)

Name R G B Name R G BAQUAMARINE 127 255 212 MAROON 128 0 0

ANTIQUE WHITE 250 235 215 MELON 227 168 105BEIGE 245 245 220 MIDNIGHT BLUE 25 25 112BLACK 0 0 0 MINT 189 252 201BLUE 0 0 255 NAVY 0 0 128

BLUE VIOLET 138 43 226 OLIVE 128 128 0BRICK 156 102 31 OLIVE DRAB 107 142 35BROWN 165 42 42 ORANGE 255 128 0

BURNT SIENNA 138 54 15 ORANGE RED 255 69 0BURNT UMBER 138 51 36 ORCHID 218 112 214CADET BLUE 95 158 160 PINK 255 192 203CHOCOLATE 210 105 30 POWDER BLUE 176 224 230COBALT 61 89 171 PURPLE 128 0 128CORAL 255 127 80 RASPBERRY 135 38 87CYAN 0 255 255 RED 255 0 0

DIMGRAY 105 105 105 ROYAL BLUE 65 105 225EMERALD GREEN 0 201 87 SALMON 250 128 114

FIREBRICK 178 34 34 SANDY BROWN 244 164 96FLESH 255 125 64 SEA GREEN 84 255 159

FOREST GREEN 34 139 34 SEPIA 94 38 18GOLD 255 215 0 SIENNA 160 82 45

GOLDENROD 218 165 32 SILVER 192 192 192GRAY 128 128 128 SKY BLUE 135 206 235GREEN 0 255 0 SLATEBLUE 106 90 205

GREEN YELLOW 173 255 47 SLATE GRAY 112 128 144HONEYDEW 240 255 240 SPRING GREEN 0 255 127HOT PINK 255 105 180 STEEL BLUE 70 130 180INDIAN RED 205 92 92 TAN 210 180 140

INDIGO 75 0 130 TEAL 0 128 128IVORY 255 255 240 THISTLE 216 191 216

IVORY BLACK 41 36 33 TOMATO 255 99 71KELLY GREEN 0 128 0 TURQUOISE 64 224 208

KHAKI 240 230 140 VIOLET 238 130 238LAVENDER 230 230 250 VIOLET RED 208 32 144LIME GREEN 50 205 50 WHITE 255 255 255MAGENTA 255 0 255 YELLOW 255 255 0

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applies paneling to an obstruction whose dimensions are 1 m by 1 m by 2 m, such that the image of thepaneling is positioned at the point (1.0,3.0,5.0). The default value of TEXTURE_ORIGIN is (0,0,0), and theglobal default can be changed by added a TEXTURE_ORIGIN statement to the MISC line.

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Chapter 8

Fire and Thermal Boundary Conditions

This chapter describes how to specify the thermal properties of solid objects. This is the most challengingpart of setting up the simulation. Why? First, for both real and simulated fires, the growth of the fireis very sensitive to the thermal properties of the surrounding materials. Second, even if all the materialproperties are known to some degree, the physical phenomena of interest may not be simulated properlydue to limitations in the model algorithms or resolution of the numerical mesh. It is your responsibility tosupply the thermal properties of the materials, and then assess the performance of the model to ensure thatthe phenomena of interest are being captured.

8.1 Basics

By default, the outer boundary of the computational domain is assumed to be a solid boundary that ismaintained at ambient temperature. The same is true for any obstructions that are added to the scene. Tospecify the properties of solids, use the namelist group SURF (Section 7.1). Starting in FDS 5, solids areassumed to consist of layers which can be made of different materials. The properties of each materialrequired are designated via the MATL namelist group (Section 8.3). These properties indicate how rapidlythe materials heat up, and how they burn. Each MATL entry in the input file must have an ID, or name, sothat they may be associated with a particular SURF via the parameter MATL_ID. For example, the input fileentries:

&MATL ID = 'BRICK'CONDUCTIVITY = 0.69SPECIFIC_HEAT = 0.84DENSITY = 1600. /

&SURF ID = 'BRICK WALL'MATL_ID = 'BRICK'COLOR = 'RED'BACKING = 'EXPOSED'THICKNESS = 0.20 /

&OBST XB=0.1, 5.0, 1.0, 1.2, 0.0, 1.0, SURF_ID='BRICK WALL' /

define a brick wall that is 4.9 m long, 1 m high, and 20 cm thick.

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The thickness of the wall indicated by the OBST line need not match that indicated by the SURF line. Thethickness of the material on the surface of the wall is dictated by the parameter THICKNESS. These twoparameters are independent for each other, the OBST line describes the overall geometric structure, the SURFline describes the characteristics of the surfaces of the geometry which includes the thickness of the layersof materials applied to that surface.

8.2 Surface Temperature and Heat Flux

This section describes how to specify simple thermal boundary conditions. These are often used when thereis little or no information about the properties of the solid materials. If the properties of the materials areknown, it is better to specify these properties and let the model compute the heat flux to, and temperatureof, the walls and other solid surfaces.

8.2.1 Specified Solid Surface Temperature

Usually, the thermal properties of a solid boundary are specified via the MATL namelist group, which is inturn invoked by the SURF entry via the character string MATL_ID. However, sometimes it is convenient tospecify a fixed temperature boundary condition, in which case set TMP_FRONT to be the surface temperaturein units of C:

&SURF ID = 'HOT WALL'COLOR = 'RED'TMP_FRONT = 200. /

Note that there is no need to specify a MATL_ID or THICKNESS. Because the wall is to be maintained at thegiven temperature, there is no need to say anything about its material composition or thickness.

8.2.2 Special Topic: Convective Heat Transfer Options

This section is labeled as a special topic because normally you do not need to modify the convective heattransfer model in FDS. However, there are special cases for which the default model may not be adequate,and this section describes some options.

Default Convective Heat Transfer Model

By default in an LES calculation, the convective heat flux to the surface is obtained from a combination ofnatural and forced convection correlations

q′′c = h ∆T W/m2 ; h = max[

C1 |∆T | 13 ,kL

C2 Re45 Pr

13

]W/m2/K (8.1)

where ∆T is the difference between the wall and the gas temperature, C1 is the coefficient for natural con-vection (1.52 for a horizontal surface and 1.31 for a vertical surface, by default) and C2 is the coefficientfor forced convection (0.037 by default, see [7] Eq. (5-85)); L is a characteristic length related to the sizeof the physical obstruction; k is the thermal conductivity of the gas, and the Reynolds Re and Prandtl Prnumbers are based on the gas flowing past the obstruction. Since the Reynolds number is proportional tothe characteristic length, L, the heat transfer coefficient is weakly related to L (for high Re, h∼ L−1/5). Forthis reason, L is taken to be 1 m for most calculations. You can change the empirical coefficients usingC_HORIZONTAL or C_VERTICAL for C1 and C_FORCED for C2, all of which are input on the MISC line.

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Changing the Convective Heat Transfer Coefficient

If you want to change the default convective heat transfer coefficient, you can set to a constant usingH_FIXED on the SURF line in units of W/m2/K. Another option is to use a turbulent convective heat transfermodel suggested by Moinuddin and Li of Victoria University, Australia, by setting H_EDDY=.TRUE. on theMISC line.

Specifying the Heat Flux at a Solid Surface

Instead of altering the convective heat transfer coefficient, you may specify a fixed heat flux directly. Twomethods are available to do this. The first is to specify a NET_HEAT_FLUX in units of kW/m2. When thisis specified FDS will compute the surface temperature required to ensure that the combined radiative andconvective heat flux from the surface is equal to the NET_HEAT_FLUX. The second method is to specifyseparately the CONVECTIVE_HEAT_FLUX, in units of kW/m2, and the radiative heat flux. The radiativeheat flux is specified by setting both TMP_FRONT and EMISSIVITY appropriately. Note that if you wishthere to be only a convective heat flux from a surface, then the EMISSIVITY should be set to zero. IfNET_HEAT_FLUX or CONVECTIVE_HEAT_FLUX is positive, the wall heats up the surrounding gases. IfNET_HEAT_FLUX or CONVECTIVE_HEAT_FLUX is negative, the wall cools the surrounding gases.

8.2.3 Special Topic: Adiabatic Surfaces

For some special applications, it is often desired that a solid surface be adiabatic, that is, there is no net heattransfer (radiative and convective) from the gas to the solid. For this case, all that must be prescribed on theSURF line is ADIABATIC=.TRUE., and nothing else. FDS will compute a wall temperature so that the sumof the net convective and radiative heat flux is zero. Specifying a surface as ADIABATIC will result in FDSdefining NET_HEAT_FLUX=0.

No solid surface is truly adiabatic; thus, the specification of an adiabatic boundary condition should be usedfor diagnostic purposes only.

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8.3 Heat Conduction in Solids

Specified temperature or heat flux boundary conditions are easy to apply, but only of limited usefulness inreal fire scenarios. In most cases, walls, ceilings and floors are made up of several layers of lining materials.The MATL namelist group is used to define the properties of the materials that make up boundary solidsurfaces. A solid boundary can consist of multiple layers1 of different materials, and each layer can consistof multiple material components.

8.3.1 Structure of Solid Boundaries

Material layers and components are specified on the SURF line via the array called MATL_ID(IL,IC).The argument IL is an integer indicating the layer index, starting at 1, the layer at the exterior boundary.The argument IC is an integer indicating the component index. For example, MATL_ID(2,3)=’BRICK’indicates that the third material component of the second layer is BRICK. In practice, the materials are oftenlisted as in the following example:

&MATL ID = 'INSULATOR'CONDUCTIVITY = 0.041SPECIFIC_HEAT = 2.09DENSITY = 229. /

&SURF ID = 'BRICK WALL'MATL_ID = 'BRICK','INSULATOR'COLOR = 'RED'BACKING = 'EXPOSED'THICKNESS = 0.20,0.10 /

Without arguments, the parameter MATL_ID is assumed to be a list of the materials in multiple layers, witheach layer consisting of only a single material component.

When a SURF is applied to the face of an OBST, the first MATL_ID is at the face of the OBST, with the otherMATL_IDs being applied in succession with the final MATL_ID being applied on the opposite face of theOBST. If in the example above, BRICK WALL was applied to the entire OBST using SURF_ID, then whendoing a heat transfer calculation from the +x face to the -x face, FDS would consider the OBST to be BRICKfollowed by INSULATOR and the same for a heat transfer calculation from the -x face to the +x face. Toavoid this the user would need to specify a second SURF that has the reverse MATL_ID and use SURF_ID6to apply the two SURF definitions to opposite faces of the OBST.

Mixtures of solid materials within the same layer can be defined using the MATL_MASS_FRACTION

keyword. This parameter has the same two indices as the MATL_ID keyword. For example, if the brick layercontains some additional water, the input could look like this:

&MATL ID = 'WATER'CONDUCTIVITY = 0.60SPECIFIC_HEAT = 4.19DENSITY = 1000. /

&SURF ID = 'BRICK WALL'MATL_ID(1,1:2) = 'BRICK','WATER'

1The maximum number of material layers is 20. The maximum number of material components is 20.

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MATL_MASS_FRACTION(1,1:2) = 0.95,0.05MATL_ID(2,1) = 'INSULATOR'COLOR = 'RED'BACKING = 'EXPOSED'THICKNESS = 0.20,0.10 / <--- for layers 1 and 2

In this example, the first layer of material, Layer 1, is composed of a mixture of brick and water. This isgiven by the MATL_ID array which specifies Component 1 of Layer 1 to be brick, and Component 2 ofLayer 1 to be water. The mass fraction of each is specified via MATL_MASS_FRACTION. In this case, brickis 95 %, by mass, of Layer 1, and water is 5 %.

It is important to notice that the components of the solid mixtures are treated as pure substances with novoids. The density of the mixture is

ρ =

(∑

i

Yi

ρi

)−1

(8.2)

where Yi are the material mass fractions and ρi are the material bulk densities defined on the MATL lines.In the example above, the resulting density of the wall would be about 1553 kg/m3. The fact that the walldensity is smaller than the density of pure brick may be confusing, but can be explained easily. If the wallcan contain water, the whole volume of the wall can not be pure brick. Instead there are voids (pores) thatare filled with water. If the water is taken away, there is only about 1476 kg/m3 of brick left. To have adensity of 1600 kg/m3 for a partially void wall, a higher density should be used for the pure brick.

8.3.2 Thermal Properties

For any solid material, specify its thermal CONDUCTIVITY (W/m·K), DENSITY (kg/m3), SPECIFIC_HEAT(kJ/kg/K), and EMISSIVITY (0.9 by default). Both CONDUCTIVITY and SPECIFIC_HEAT can be functionsof temperature. DENSITY and EMISSIVITY cannot. Temperature-dependence is specified using the RAMPconvention. As an example, consider marinite, a wall material suitable for high temperature applications:

&MATL ID = 'MARINITE'EMISSIVITY = 0.8DENSITY = 737.SPECIFIC_HEAT_RAMP = 'c_ramp'CONDUCTIVITY_RAMP = 'k_ramp' /

&RAMP ID='k_ramp', T= 24., F=0.13 /&RAMP ID='k_ramp', T=149., F=0.12 /&RAMP ID='k_ramp', T=538., F=0.12 /&RAMP ID='c_ramp', T= 93., F=1.172 /&RAMP ID='c_ramp', T=205., F=1.255 /&RAMP ID='c_ramp', T=316., F=1.339 /&RAMP ID='c_ramp', T=425., F=1.423 /

Notice that with temperature-dependent quantities, the RAMP parameter T means Temperature, and F is thevalue of either the specific heat or conductivity. In this case, neither CONDUCTIVITY nor SPECIFIC_HEATis given on the MATL line, but rather the RAMP names.

Prior to FDS5, the thermal radiation from the gas space was always absorbed at the surface of the solidmaterial and the emission to the gas space took place on the surface. Starting in FDS5, the solid material canbe given an ABSORPTION_COEFFICIENT (1/m) that allows the radiation penetrate and absorb into the solid.Correspondingly, the emission of the material is based on the internal temperatures, not just the surface.

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8.3.3 Back Side Boundary Conditions

The layers of a solid boundary are listed in the order from the surface to the interior. By default, this inner-most layer is assumed to back up to an air gap at ambient temperature. This is true even if the obstructionforms a wall in the model that backs up to another compartment. A good example of the default back sideboundary condition is a sheet of gypsum board attached to wood studs. It is assumed that the back side ofthe gypsum board is an ambient temperature void space within the wall. It does not matter if the obstructionon which the boundary condition is applied is thick or thin.

There are other back side boundary conditions that can be applied. One is to assume that the wallbacks up to an insulated material in which case no heat is lost to the backing material. The expressionBACKING=’INSULATED’ on the SURF line prevents any heat loss from the back side of the material. Useof this condition means that you do not have to specify properties of the inner insulating material because itis assumed to be perfectly insulated.

If the wall is assumed to back up to the room on the other side of the wall and you want FDS to calculatethe heat transfer through the wall into the space behind the wall, the attribute BACKING=’EXPOSED’ shouldbe listed on the SURF line. This feature only works if the wall is less than or equal to one mesh cell thick,and if there is a non-zero volume of computational domain on the other side of the wall. Obviously, if thewall is an external boundary of the domain, the heat is lost to an ambient temperature void.

8.3.4 Initial and Back Side Temperature

By default, the initial temperature of the solid material is set to ambient (TMPA on the MISC line). UseTMP_INNER on the SURF line to specify a different initial temperature. Also, the back side temperatureboundary condition of a solid can be set using the parameter TMP_BACK on the SURF line. TMP_BACK is notthe actual back side surface temperature, but rather the gas temperature to which the back side surface is ex-posed. This parameter has no meaning for surfaces with BACKING=’EXPOSED’ or BACKING=’INSULATED’.

Note that the parameters TMP_INNER and TMP_BACK are only meaningful for solids with specifiedTHICKNESS and material properties (via the MATL_ID keyword).

8.3.5 Walls with Different Materials Front and Back

If you apply the attribute BACKING=’EXPOSED’ on a SURF line that is applied to a zero or one-cell thickobstruction, FDS calculates the heat conduction through the entire THICKNESS and it uses the gas phasetemperature and heat flux on the front and back sides for boundary conditions. A redundant calculation isperformed on the opposite side of the obstruction, so be careful how you specify multiple layers. If thelayering is symmetric, the same SURF line can be applied to both sides. However, if the layering is notsymmetric, you must create two separate SURF lines and apply one to each side. For example, a hollowbox column that is made of steel and covered on the outside by a layer of insulation material and a layer ofplastic on top of the insulation material, would have to be described with two SURF lines like the following:

&SURF ID = 'COLUMN EXTERIOR'COLOR = 'ANTIQUE WHITE'BACKING = 'EXPOSED'MATL_ID(1:3,1) = 'PLASTIC','INSULATION','STEEL'THICKNESS(1:3) = 0.002,0.036,0.0063 /

&SURF ID = 'COLUMN INTERIOR'COLOR = 'BLACK'BACKING = 'EXPOSED'

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MATL_ID(1:3,1) = 'STEEL','INSULATION','PLASTIC'THICKNESS(1:3) = 0.0063,0.036,0.002 /

If, in addition, the insulation material and plastic are combustible, and their burning properties are specifiedon the appropriate MATL lines, then you need to indicate which side of the column would generate the fuelvapor. In this case, the steel is impermeable; thus you should add the parameter LAYER_DIVIDE=2.0 tothe SURF line labeled ’COLUMN EXTERIOR’ to indicate that fuel vapors formed by the heating of the twofirst layers (’PLASTIC’ and ’INSULATION’) are to be driven out of that surface. You need to also specifyLAYER_DIVIDE=0.0 on the SURF line labeled ’COLUMN INTERIOR’ to indicate that no fuel vapors areto driven into the interior of the column. In fact, values from 0.0 to 1.0 would work equally because thematerial ’STEEL’ would not generate any fuel vapors.

By default, LAYER_DIVIDE is 0.5 times the number of layers for surfaces with EXPOSED backing, andequal to the number of layers for other surfaces.

8.3.6 Special Topic: Non-Planar Walls and Targets

All obstructions in FDS are assumed to conform to the rectilinear mesh, and all bounding surfaces areassumed to be flat planes. However, many objects, like cables, pipes, and ducts, are not flat. Even thoughthese objects have to be represented in FDS as “boxes,” you can still perform the internal heat transfercalculation as if the object were really cylindrical or spherical. For example, the input lines:

&OBST XB=0.0,5.0,1.1,1.2,3.4,3.5, SURF_ID='CABLE' /&SURF ID='CABLE', MATL_ID='PVC', GEOMETRY='CYLINDRICAL', THICKNESS=0.01 /

can be used to model a power cable that is 5 m long, cylindrical in cross section, 2 cm in diameter. Theheat transfer calculation is still one-dimensional; that is, it is assumed that there is a uniform heat fluxall about the object. This can be somewhat confusing because the cable is represented as an obstruction ofsquare cross section, with a separate heat transfer calculation performed at each face, and no communicationamong the four faces. Obviously, this is not an ideal way to do solid phase heat transfer, but it does provide areasonable bounding surface temperature for the gas phase calculation. More detailed assessment of a cablewould require a two or three-dimensional heat conduction calculation, which is not included in FDS. UseGEOMETRY=’SPHERICAL’ to describe a spherical object.

8.3.7 Special Topic: Solid Phase Numerical Gridding Issues

To compute the temperature and reactions inside the solids, FDS solves the one-dimensional heat transferequation numerically. The size of the mesh cells on the surface of the solid is automatically chosen using arule that makes the cell size smaller than the square root of the material diffusivity (k/ρc). By default, thesolid mesh cells increase towards the middle of the material layer and are smallest on the layer boundaries.

The default parameters are usually appropriate for simple heat transfer calculations but sometimes theuse of pyrolysis reactions makes the temperatures and burning rate fluctuate. The numerical stability of thesolid phase solution may then be improved by making the mesh density more uniform inside the materialand by making the mesh cells smaller. Adjustments may also be needed in case of extremely transient heattransfer situations. Use STRETCH_FACTOR=1. on the SURF line to have a perfectly uniform mesh. Valuesbetween 1 and 2 give different levels of stretching. The size of all the mesh cells can be scaled by settingCELL_SIZE_FACTOR less than 1.0. For example, CELL_SIZE_FACTOR=0.5 makes the mesh cells half thesize. Setting WALL_INCREMENT=1 on the TIME line forces the solid phase temperatures to be solved onevery time step.

If all the material components of the surface are reacting, and the pyrolysis reactions have no solidresidue, the thickness of the surface is going to shrink when the surface reacts. When all the material of a

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shrinking surface is consumed but BURN_AWAY is not prescribed, the surface temperature is set to TMP_BACK,convective heat flux to zero and burning rate to zero.

See Section 8.5 for ways to check and improve the accuracy of the solid phase calculation.

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8.4 Pyrolysis Models

FDS has several approaches for describing the pyrolysis of solids and liquids. The approach to take dependslargely on the availability of material properties and the appropriateness of the underlying pyrolysis model.This section provides a description of the parameters that describe a burning solid material when the burningrate is known. In other words, you use these parameters to specify the burning rate.

8.4.1 A Gas Burner with a Specified Heat Release Rate

Solids and liquid fuels can be modeled by specifying their relevant properties via the MATL namelist group.However, if you simply want to specify a fire of a given heat release rate (HRR), you need not specify anymaterial properties. A specified fire is basically modeled as the ejection of gaseous fuel from a solid surfaceor vent. This is essentially a burner, with a specified Heat Release Rate Per Unit Area, HRRPUA, in units ofkW/m2. For example

&SURF ID='FIRE',HRRPUA=500. /

applies 500 kW/m2 to any surface with the attribute SURF_ID=’FIRE’. See the discussion of Time Depen-dent Conditions in Section 10 to learn how to ramp the heat release rate up and down.

An alternative to HRRPUA with the exact same functionality is MLRPUA, except this parameter specifiesthe Mass Loss Rate of fuel gas Per Unit Area in kg/m2/s. Do not specify both HRRPUA and MLRPUA onthe same SURF line. With either, the stoichiometry of the gas phase reaction is set by the parameters onthe REAC line. All of the species associated with the combustion process are accounted for by way of themixture fraction variable and should not be explicitly prescribed. The exception to this rule is where a non-reacting gas is introduced into the domain that merely serves as a diluent, like CO2 from an extinguisher orH2O from evaporated sprinkler droplets (see Section 11.2 for details). If a finite rate combustion model isdesired instead of the default mixture fraction model, see Section 11.2.4.

Specifying HRRPUA or MLRPUA automatically invokes the mixture fraction combustion model.

8.4.2 Special Topic: A Radially-Spreading Fire

Sometimes it is desired that a fire spread radially at some specified rate. Rather than trying to design materialproperties to achieve this, you can alternatively use a VENT in a special way. If the SURF_ID associated witha VENT defines a specified heat release rate, HRRPUA, and time history, RAMP_Q or TAU_Q, you can alsospecify XYZ and SPREAD_RATE on the VENT line. Then the fire is directed to start at the point XYZ andspread radially at a rate of SPREAD_RATE (m/s). The ramp-up begins at the time when the fire arrives at agiven point. For example, the lines

&SURF ID='FIRE', HRRPUA=500.0, RAMP_Q='fireramp' /&RAMP ID='fireramp', T= 0.0, F=0.0 /&RAMP ID='fireramp', T= 1.0, F=1.0 /&RAMP ID='fireramp', T=30.0, F=1.0 /&RAMP ID='fireramp', T=31.0, F=0.0 /&VENT XB=0.0,5.0,1.5,9.5,0.0,0.0, SURF_ID='FIRE', XYZ=1.5,4.0,0.0, SPREAD_RATE=0.03 /

create a rectangular patch at z = 0 on which the fire starts at the point (1.5,4.0,0.0) and spreads outwards at arate of 0.03 m/s. Each surface cell burns for 30 s as the fire spreads outward, creating a widening ring of fire.Note that the RAMP_Q is used in this construct to turn the burning on and off to simulate the consumptionof fuel as the fire spreads radially. It should not be used to mimic the “t-squared” curve – the whole point

65

of the exercise is to mimic this curve in a more natural way. Eventually, the fire goes out as the ring growspast the boundary of the rectangle. Some trial and error is probably required to find the SPREAD_RATE thatleads to a desired time history of the heat release rate.

8.4.3 Solid Fuels that Burn at a Specified Rate

Real objects, like furnishings, office equipment, and so on, are often difficult to describe via the SURF andMATL parameters. Sometimes the only information about a given object is its bulk thermal properties, its“ignition” temperature, and what its subsequent burning rate is, as a function of time from ignition. For thissituation, add lines similar to the following:

&MATL ID = 'stuff'CONDUCTIVITY = 0.1SPECIFIC_HEAT = 1.0DENSITY = 900.0 /

&SURF ID = 'my surface'COLOR = 'GREEN'MATL_ID = 'stuff'HRRPUA = 1000.IGNITION_TEMPERATURE = 500.RAMP_Q = 'fire_ramp'THICKNESS = 0.01 /

&RAMP ID='fire_ramp', T= 0.0, F=0.0 /&RAMP ID='fire_ramp', T= 10.0, F=1.0 /&RAMP ID='fire_ramp', T=310.0, F=1.0 /&RAMP ID='fire_ramp', T=320.0, F=0.0 /

An object with surface properties defined by ’my surface’ shall burn at a rate of 1000 kW/m2 after alinear ramp-up of 10 s following its “ignition” when its surface temperature reaches 500 C. Burning shallcontinue for 5 min, and then ramp-down in 10 s. Note that the time T in the RAMP means time from ignition.Note also that now the ”ignition temperature” is a surface property, not material property.

After the surface has ignited, the heat transfer into the solid is still being solved but there is no couplingbetween the burning rate and the surface temperature. As a result, the surface temperature may increase toomuch. To account for the energy loss due to the vaporization of the solid fuel, HEAT_OF_VAPORIZATIONcan be specified for the surface. For example, when using the lines below, the net heat flux at the materialsurface is reduced by a factor 1000 kJ/kg times the instantaneous burning rate.

&SURF ID = 'my surface'COLOR = 'GREEN'MATL_ID = 'stuff'HRRPUA = 1000.IGNITION_TEMPERATURE = 500.HEAT_OF_VAPORIZATION = 1000.RAMP_Q = 'fire_ramp'THICKNESS = 0.01 /

The parameters HRRPUA, IGNITION_TEMPERATURE, and HEAT_OF_VAPORIZATION are all telling FDSthat you want to control the burning rate yourself, but you still want to simulate the heating up and “ignition”of the fuel. When these parameters appear on the SURF line, they are acting in concert. If HRRPUA appearsalone, the surface will begin burning at the start of the simulation, like a piloted burner. The addition ofan IGNITION_TEMPERATURE delays burning until your specified temperature is reached. The addition of

66

HEAT_OF_VAPORIZATION tells FDS to account for the energy used to vaporize the fuel. For any of theseoptions, if a MATL line is invoked by a SURF line containing a specified HRRPUA, then that MATL ought tohave only thermal properties. It should have no reaction parameters, product yields, and so on, like thosedescribed in the previous sections. By specifying HRRPUA, you are controlling the burning rate rather thanletting the material pyrolyze based on the conditions of the surrounding environment.

8.4.4 Solid Fuels that do NOT Burn at a Specified Rate

This section describes the parameters that describe the reactions that occur within solid materials when theyare burning. It is strongly recommended before reading this section that you read some background materialon solid phase pyrolysis, for example “Thermal Decomposition of Polymers,” by Hirschler and Morgan, or“Flaming Ignition of Solid Fuels,” by Torero, both of which are in the 4th edition of the SFPE Handbook ofFire Protection Engineering.

The Reaction Mechanism

A solid surface in FDS can consist of multiple layers with multiple material components per layer. Thematerial components are described via MATL lines and are specified on the SURF line that describes thestructure of the solid. Each MATL can undergo several reactions that may occur at different temperatures. Itmay not undergo any – it may only just heat up. However, if it is to change form via one or more reactions,designate the number of reactions with the integer N_REACTIONS. It is very important that you designateN_REACTIONS or else FDS will ignore all parameters associated with reactions. Note that experimentalevidence of multiple reactions does not imply that a single material is undergoing multiple reactions, butrather that multiple material components are undergoing individual reactions at distinct temperatures. Cur-rently, the maximum number of reactions for each material is 10 and the chain of consecutive reactions maycontain up to 20 steps.

For a given MATL, the jth reaction can produce a (single) solid whose name is RESIDUE(j), plus watervapor, and/or fuel gas. For example, the evaporation of water from a solid material is described by the“reaction” that converts liquid water to water vapor. This reaction occurs in the neighborhood of 100 Cand produces only water vapor. It does not produce a solid RESIDUE nor fuel gas. However, a pyrolyzingsolid might undergo a reaction that produces a solid RESIDUE, water vapor, and fuel gas, all as part of thesingle reaction step. This information is conveyed to FDS via the yields: NU_RESIDUE(j), NU_WATER(j),and NU_FUEL(j), respectively. The integer j indicates to which reaction the parameters apply. If, like theevaporation of water, only water vapor is produced, set NU_WATER(j)=1.0 and the other two to zero. Theyields are all zero by default. If NU_RESIDUE(j) is non-zero, then you must indicate what the solid residueis via RESIDUE(j), the ID of another MATL that is also listed in the input file. Ideally, the sum of theyields should add to 1, meaning that the mass of the reactant is conserved. However, there are times whenit is convenient to have the yields sum to something less than one. For example, the spalling or ablation ofconcrete can be described as a “reaction” that consumes energy but does not produce any “product” becausethe concrete is assumed to have either fallen off the surface in chunks or pulverized powder. The concrete’smass is not conserved in the model because it has essentially disappeared from that particular surface. Amore general way to specify gaseous yields is to use the parameter NU_GAS, as explained in Section 11.2.3.

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The Reaction Rates

For each reaction that each material component undergoes you must specify kinetic parameters of the reac-tion rate. The general evolution equation for a material undergoing one or more reactions is:

∂Ys,i

∂t=−

Nr,i

∑j=1

ri j +Nm

∑i′=1

Nr,i′

∑j=1

νs,i′ j ri′ j (i′ 6= i) ; ri j = Ai j Y ns,i js,i exp

(− Ei j

RTs

); Ys,i =

(ρs,i

ρs0

)(8.3)

The term, ri j, defines the rate of reaction at the temperature, Ts, of the ith material undergoing its jth reaction.The second term on the right of the equation represents the contributions of other materials producing theith material as a residue with a yield of νs,i′ j. This term is denoted by NU_RESIDUE(j) on the i′-th MATL

line. ρs,i is the density of the ith material component of the layer, defined as the mass of the ith materialcomponent divided by the volume of the layer. ρs0 is the initial density of the layer. Thus, Ys,i = ρs,i/ρs0 isa quantity that increases if the ith material component is produced as a residue of some other reaction, ordecreases if the ith component decomposes. If the layer is composed of only one material, then ρs,i/ρs0 isinitially 1. ns,i j is the reaction order and prescribed under the name N_S(j), and is 1 by default. If the valueof ns is not known, it is a good starting point to assume ns = 1.

The pre-exponential factor, Ai j, is prescribed under the name A(j) on the MATL line of the ith material,with units of s−1. Ei j, the activation energy, is prescribed via E(j) in units of kJ/kmol. Remember that1 kcal is 4.184 kJ, and be careful with factors of 1000. For a given reaction, specify both A and E, or neither.Do not specify only one of these two parameters. Typically, these parameters only have meaning when bothare derived from a common set of experiments, like TGA (Thermo-Gravimetric Analysis).

It is very important to keep in mind that A and E are not available for most real materials. If A and Eare not known, there are several parameters that can be used by FDS to derive effective values. The mostimportant parameter to specify in place of A and E is the REFERENCE_TEMPERATURE (C). To understandthis parameter, consider the plot shown in Fig. 8.1. These curves represent the results of a hypothetical TGAexperiment. The Mass Fraction (blue curve) is the normalized density of the material (Y ) which decreasesas the sample is slowly heated, in this case at a rate of 5 K/min. The Reaction Rate (green curve) is therate of change of the mass fraction as a function of time (−dY/dt). Where this curve peaks is referredto in FDS as the REFERENCE_TEMPERATURE.2 Note that the REFERENCE_TEMPERATURE is not the sameas an ignition temperature, nor is it necessarily the surface temperature of the burning solid. Rather, it issimply the temperature at which the mass fraction of the material decreases at its maximum rate within thecontext of a TGA or similar experimental apparatus. The kinetic constants for the reaction are found fromthe formulae3:

E =erp

Y0

RT 2p

T; A =

erp

Y0eE/RTp (8.4)

where Tp and rp/Y0 are the reference temperature and rate, respectively. The REFERENCE_RATE is thereaction rate, in units of s−1, at the given REFERENCE_TEMPERATURE divided by the mass fraction, Y0, ofmaterial in the original sample undergoing the reaction. For a single component, single reaction material,Y0 = 1. The HEATING_RATE (T ) is the rate at which the temperature of the TGA (or equivalent) testapparatus was increased. It is input into FDS in units of K/min (in the formula, it is expressed in K/s). Itsdefault value is 5 K/min. In Fig. 8.1, the area under the green curve (Reaction Rate) is equal to the heatingrate (in units of K/s).

There are many cases where it is only possible to estimate the REFERENCE_TEMPERATURE (Tp) of a par-ticular reaction because micro-scale calorimetry data is unavailable. In such cases, an additional parameter

2The term “reference temperature” is used simply to maintain backward compatibility with earlier versions of FDS.3These formulas have been derived from an analysis that considers a first-order reaction. When using the proposed method, do

not specify non-unity value for the reaction order N_S on the MATL line.

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0 50 100 150 200 250 300 350 4000

0.2

0.4

0.6

0.8

1

Temperature (C)

Mas

sFr

action

0 50 100 150 200 250 300 350 4000

0.4

0.8

1.2

1.6

2

Rea

ctio

nR

ate

(s−

1)×

103 dY

dt=−AY exp(−E/RT ) Y (0) = 1

Tp = 300 C

rp = 0.002 s−1

T = 5 K/min

νs = 0

Figure 8.1: The blue curve represents the normalized mass, Y = ρs/ρs0, of a solid material undergoingheating at a rate of 5 K/min. The green curve represents the reaction rate, −dY/dt. The system of ordinarydifferential equations that describe the transformation is shown at right. Note that the parameters Tp, rp, andνs represent the “reference” temperature, reaction rate, and residue yield of the single reaction. From theseparameters, values of A and E can be estimated using the formulae in (8.4).

can be specified along with REFERENCE_TEMPERATURE (Tp) to help fine tune the shape of the reaction ratecurve, assuming some sort of measurement or estimate has been made to indicate at what temperature, andover what temperature range, the reaction takes place. The PYROLYSIS_RANGE (∆T ) is the approximatewidth (in degrees Celsius or Kelvin) of the green curve, assuming its shape to be roughly triangular. Itsdefault value is 80 C. Using these input parameters, an estimate is made of the peak reaction rate, rp, withwhich E, then A, are calculated.

rp

Y0=

2 T∆T

(1−νs) (8.5)

The parameter, νr, is the yield of solid residue.When in doubt about the values of these parameters, just specify the REFERENCE_TEMPERATURE. Note

that FDS will automatically calculate A and E using the above formulae. Do not specify A and E if you spec-ify REFERENCE_TEMPERATURE, and do not specify PYROLYSIS_RANGE if you specify REFERENCE_RATE.For the material decomposition shown in Fig. 8.1, the MATL would have the form:

&MATL ID = 'My Fuel'...HEAT_OF_COMBUSTION = ...N_REACTIONS = 1NU_FUEL(1) = 1.NU_RESIDUE(1) = 0.REFERENCE_TEMPERATURE(1) = 300.REFERENCE_RATE(1) = 0.002HEATING_RATE(1) = 5.HEAT_OF_REACTION(1) = ... /

Note that the argument (1) has been added to the reaction parameters to emphasize the fact that theseparameters are stored in arrays of length equal to N_REACTIONS. If there is only one reaction, you need notinclude the (1), but it is a good habit to get into. Note also that the HEAT_OF_COMBUSTION is the energyreleased per unit mass of fuel gas that mixes with oxygen and combusts. This has nothing to do with the

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pyrolysis process, so why is it specified here? The answer is that there can be only one gas phase reaction offuel and oxygen in FDS, but there can be dozens of different materials and dozens of solid phase reactions.To ensure that the fuel vapors from different materials combust to produce the proper amount of energy, it isvery important to specify a HEAT_OF_COMBUSTION for each material. That way, the mass loss rate of fuelgases is automatically adjusted so that the effective mass loss rate multiplied by the single, global, gas phaseheat of combustion produces the expected heat release rate. If, for example, the HEAT_OF_COMBUSTION

specified on the REAC line is twice that specified on the MATL line, the mass of contained within wall cellwill be decremented by that determined by the pyrolysis model, but the mass added to gas phase wouldbe reduced by 50 %. A different value of heat of combustion can be specified for each reaction and eachgaseous species using the notation HEAT_OF_COMBUSTION(j,s), as explained in Section 11.2.3.

Note that versions of FDS from 5.0 through 5.3 used a slightly different definition ofREFERENCE_TEMPERATURE and REFERENCE_RATE. This will lead to different results when using the dif-ferent versions of the model.

Multiple Solid Phase Reactions

The solid phase reaction represented by Fig. 8.1 is very simple – a single, homogenous material is heated andgasified completely. In general, real materials are not so simple. First, they consist of more than one materialcomponent, each of which can react over a particular temperature interval, and some of which leave behind asolid residue. Some material components may even undergo multiple reactions that form different residues,like woods that form various amounts of tar, char, and ash, depending on the rate of heating. Figure 8.2demonstrates a more complicated material than the one previously described. It is a hypothetical materialthat contains 10 % (by mass) water and 90 % solid material. The water evaporates in the neighborhood of100 C and the solid pyrolyzes in the neighborhood of 300 C, leaving 20 % of its mass behind in the formof a solid residue.

The key input lines for this reaction are shown in Fig. 8.3. Note that the only parameters shown arethose that describe the reaction mechanism, and that each of these parameters can be found either fromvisual inspection of the the mass loss (blue) curve or the reaction rate (green) curve. Even if TGA or similardata were unavailable in this case, you can still model the solid as a combination of water that evaporatesat 100 C and some other material that pyrolyzes in the vicinity of 300 C, leaving 20 % of its mass as aresidue. The full set of parameters for these cases are listed in pyrolysis_1.fds and pyrolysis_2.fds. Thoseinterested in testing potential solid phase reaction mechanisms ought to use these test cases as templates.

The Heat of Reaction

Equation (8.3) describes the rate of the reaction as a function of temperature. Most solid phase reactionsrequire energy; that is, they are endothermic. The amount of energy consumed, per unit mass of reactantthat is converted into something else, is specified by the HEAT_OF_REACTION(j). Technically, this isthe enthalpy difference between the products and the reactant. A positive value indicates that the reactionis endothermic; that is, the reaction takes energy out of the system. Usually the HEAT_OF_REACTION isaccurately known only for simple phase change reactions like the vaporization of water. For other reactions,it must be determined empirically.

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0 50 100 150 200 250 300 350 4000

0.2

0.4

0.6

0.8

1

Temperature (C)

Mas

sFr

action

0 50 100 150 200 250 300 350 4000

0.4

0.8

1.2

1.6

2

Rea

ctio

nR

ate

(s−

1)×

103

dY1

dt=−A1,1 Y1 exp(−E1,1/RT ) Y1(0) = 0.1

dY2

dt=−A2,1 Y2 exp(−E2,1/RT ) Y2(0) = 0.9

dY3

dt=−νs,2,1

dY2

dtY3(0) = 0.0

Tp,1,1 = 100+273 K Tp,2,1 = 300+273 K

rp,1,1 = 0.0016 s−1 rp,2,1 = 0.0012 s−1

νs,1,1 = 0 νs,2,1 = 0.2

T = 5 K/min

Figure 8.2: The blue curve represents the combined mass fraction, ∑Yi, and the green curve the net reactionrate, −d/dt(∑Yi), for a material that contains 10 % water (by mass) that evaporates at a temperature of100 C, and 90 % solid material that pyrolyzes at 300 C, leaving a 20 % (by mass) residue behind. Notethat the numbered subscripts refer to the material component and the reaction, respectively. In this case,there are three material components, and the first two each undergo a single reaction. The third materialcomponent is formed as a residue of the reaction of the second material. The system of ordinary differentialequations that governs the transformation of the materials is shown at right.

Special Topic: The “Threshold” Temperature

In FDS, the reaction rate expression in Eq. (8.3) includes an optional term:

ri j = Ai j Y ns,i js,i exp

(− Ei j

RTs

)max

[0,Ts−Tthr,i j

]nt,i j (8.6)

Tthr,i j is an optional “threshold” temperature that allows the definition of non-Arrhenius pyrolysis functionsand ignition criteria, and is prescribed by THRESHOLD_TEMPERATURE(j). By default, Tthr,i j is -273.15degrees Celsius, nt, j is zero; thus, the last term of Equation 8.6 does not affect the pyrolysis rate. The termcan be used to describe a threshold temperature for the pyrolysis reaction by setting Tthr,i j and nt, j = 0. Thenthe term is equal to 0 at temperatures below Tthr,i j and 1 at temperatures above. nt, j is prescribed under thename N_T(j).

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&SURF ID = 'SAMPLE'...MATL_ID(1,1:2) = 'stuff','water'MATL_MASS_FRACTION(1,1:2) = 0.9,0.1 /

&MATL ID = 'water'EMISSIVITY = 1.0DENSITY = 1000.CONDUCTIVITY = 0.20SPECIFIC_HEAT = 4.184N_REACTIONS = 1REFERENCE_TEMPERATURE = 100.PYROLYSIS_RANGE = 10.HEATING_RATE = 5.NU_WATER = 1.HEAT_OF_REACTION = 2500. /

&MATL ID = 'stuff'EMISSIVITY = 1.0DENSITY = 500.CONDUCTIVITY = 0.20SPECIFIC_HEAT = 1.0N_REACTIONS = 1REFERENCE_TEMPERATURE = 300.PYROLYSIS_RANGE = 80.HEATING_RATE = 5.NU_FUEL = 0.8NU_RESIDUE = 0.2RESIDUE = 'ash'HEAT_OF_REACTION = 1000. /

&MATL ID = 'ash'EMISSIVITY = 1.0DENSITY = 500.CONDUCTIVITY = 0.20SPECIFIC_HEAT = 1.0 /

Figure 8.3: Input parameters for sample case pyrolysis_2.

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8.4.5 Liquid Fuels

For a liquid fuel, the thermal properties are similar to those of a solid material, with a few exceptions. Theevaporation rate of the fuel is governed by the Clausius-Clapeyron equation (see FDS Technical ReferenceGuide for details). The drawback of this approach is that the fuel mass flux is not an explicit function oftemperature, but rather an iterative result depending on the temperature and flow conditions. To initiate theevaporation, an initial value of the fuel vapor volume flux is needed. If the initial value is (relatively) high,the evaporation starts regardless of any ignition source, and the the fuel begins burning at once.

Figure 8.4 contains the key input parameters to describe a steel pan filled with a thin layer of ethanol.Note that the material properties are not all traceable to a measurement.

&MATL ID = 'ETHANOL LIQUID'EMISSIVITY = 1.0NU_FUEL = 0.97HEAT_OF_REACTION = 880.CONDUCTIVITY = 0.17SPECIFIC_HEAT = 2.45DENSITY = 787.ABSORPTION_COEFFICIENT = 40.BOILING_TEMPERATURE = 76. /

&MATL ID = 'STEEL'EMISSIVITY = 1.0DENSITY = 7850.CONDUCTIVITY = 45.8SPECIFIC_HEAT = 0.46 /

&MATL ID = 'CONCRETE'DENSITY = 2200.CONDUCTIVITY = 1.2SPECIFIC_HEAT = 0.88 /

&SURF ID = 'ETHANOL POOL'FYI = '4 kg of ethanol in a 0.7 m x 0.8 m pan'COLOR = 'YELLOW'MATL_ID = 'ETHANOL LIQUID','STEEL','CONCRETE'THICKNESS = 0.0091,0.001,0.05TMP_INNER = 18. /

Figure 8.4: Input parameters for sample case ethanol_pan.

The inclusion of BOILING_TEMPERATURE on the MATL line tells FDS to use its liquid pyrolysis model.It also automatically sets N_REACTIONS=1, that is, the only “reaction” is the phase change from liquidto gaseous fuel. Thus, HEAT_OF_REACTION in this case is the latent heat of vaporization. The gaseousfuel yield, NU_FUEL, is 0.97 instead of 1 to account for impurities in the liquid that do not take part in thecombustion process.

The thermal conductivity, density and specific heat are used to compute the loss of heat into the liquidvia conduction using the same one-dimensional heat transfer equation that is used for solids. Obviously, theconvection of the liquid is important, but is not considered in the model.

The initial value of the fuel vapor volume flux can be specified using the parameter INITIAL_VAPOR_FLUX.Its default value is 5 ·10−4 m3/(sm2).

Note also the ABSORPTION_COEFFICIENT for the liquid. This denotes the absorption in depth ofthermal radiation. Liquids do not just absorb radiation at the surface, but rather over a thin layer near the

73

surface. Its effect on the burning rate is significant.

In the current implementation of the liquid fuel model, the evaporation rate is strongly grid dependent. Thus,it should be used with caution.

8.4.6 Fuel Burnout

The thermal properties of a solid or liquid fuel determine the length of time for which it can burn. In general,the burnout time is a function of the mass loss rate, m′′, the density, ρs, and the layer thickness, δs:

tb =ρs δs

m′′(8.7)

However, each type of pyrolysis model handles fuel burnout in a slightly different way. These differenceswill be highlighted in the individual sections below.

Solid Fuel Burnout

If a heat release rate RAMP function is not included for a solid fuel that burns at a specified rate, the surfacewill continue to burn at the specified rate indefinitely with no fuel burnout. If detailed heat release rate versustime data is not available, you can estimate the burnout time for a surface using the heat of combustion, ∆H,material density, ρs, material thickness, δs, and HRRPUA, q′′f :

tb =ρs δs ∆H

q′′f(8.8)

Use the RAMP function to stop the burning once the calculated burnout time is reached.The burnout time of a reacting solid fuel is calculated automatically by FDS based on the layer THICKNESS,

component DENSITY, and the calculated burning rate.

Liquid Fuel Burnout

The burnout time of a liquid fuel is calculated automatically based on the liquid layer THICKNESS, liquidDENSITY, and the calculated burning rate.

Special topic: Making Fuels Disappear (BURN_AWAY)

If a burning object is to disappear from the calculation once it is consumed, set BURN_AWAY=.TRUE. on thecorresponding SURF line. The solid object disappears from the calculation cell by cell, as the mass containedby each mesh cells is consumed either by the pyrolysis reactions or by the prescribed HRR. The mass ofeach mesh cell is the cell face area multiplied by the surface density of the SURF type.

The following issues should be kept in mind when using BURN_AWAY:

• For reacting surfaces, the surface density is computed as a sum of the layer densities multiplied by thelayer thicknesses. This value can be over-ridden by setting SURFACE_DENSITY on the SURF line. Forsurfaces with prescribed HRR (HRRPUA), SURFACE_DENSITY parameter is the only way of defining themass of the object.

• Use BURN_AWAY parameter cautiously. If an object has the potential of burning away, a significantamount of extra memory has to be set aside to store additional surface information as the rectangularblock is eaten away.

74

• If BURN_AWAY is prescribed, the SURF should be applied to the entire object, not just a face of the objectbecause it is unclear how to handle edges of solid obstructions that have different SURF_IDs on differentfaces.

• If the volume of the obstruction changes because it has to conform to the uniform mesh, FDS does notadjust the burning rate to account for this as it does with various quantities associated with areas, likeHRRPUA.

• A parameter called BULK_DENSITY (kg/m3) can be applied to the OBST rather than the SURF line. Thisparameter is used to determine the combustible mass of the solid object. The calculation uses the user-specified object dimensions, not those of the mesh-adjusted object. This parameter over-rides all otherparameters from which a combustible mass would be calculated.

• The mass of the object is based on the densities of the all material components (MATL), but it is onlyconsumed by mass fluxes of the known species. If the sum of the gaseous yields (Section 11.2.3) is lessthan one, it will take longer to consume the mass.

Example Case: box_burn_away

This is a silly example of a solid block of “foam” that is pyrolyzed until it is completely consumed. Theheat flux is generated by placing hot surfaces around the box. There is no combustion. In the first example(box_burn_away), the released gas is fuel (mixture fraction) and in the second example (box_burn_away2)it is an additional species called ’GAS’. The properties of the block of foam were chosen simply to assurea quick calculation. The objective of the test is to check that the released mass and the integrated burningrate is consistent with the material properties of the block. The block is 0.4 m on a side, with a density of20 kg/m3. The integrated densities of the pyrolysis product gases (written to box_burn_away_devc.csv andbox_burn_away2_devc.csv), as well as the integrated burning rate (written to box_burn_away_hrr.csv) inthe end of the 30 s calculation ought to be:

(0.4)3 m3×20 kg/m3 = 1.28 kg (8.9)

0 5 10 15 20 25 300

0.5

1

1.5

Time (s)

Mas

s(k

g)

Pyrolyzed Mass (box burn away)

Analytical massFDS (fuel)

0 5 10 15 20 25 300

0.5

1

1.5

Time (s)

Mas

s(k

g)

Pyrolyzed Mass (box burn away2)

Analytical massFDS (GAS)

Figure 8.5: Output of box_burn_away test cases.

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8.5 Testing Your Pyrolysis Model

Real materials that can burn can be very complicated. Undoubtedly, the SURF and MATL lines in the inputfile will consist of a combination of empirical and fundamental properties, often originating from differentsources. How do you know that the various property values and the associated thermo-physical model inFDS constitute an appropriate description of the solid? For a full-scale simulation, it is hard to untangle theuncertainties associated with the gas and solid phase routines. However, it is easy to perform a simple checkof any set of surface properties by essentially turning off the gas phase – no combustion and no convectiveheat transfer. There are several parameters that allow you do this, spread out over the various namelistgroups.

1. Create a trivially small mesh, just to let FDS run. Since the gas phase calculation is essentially beingshut off, you just need 4 cells in each direction (IJK=4,4,4) for the pressure solver to function properly.

2. On the TIME line, set WALL_INCREMENT=1 to force FDS to update the solid phase every time step(normally it does this every other time step), and set DT to whatever value appropriate for the solidphase calculation. Since there is no gas phase calculation that will limit the time step, it is best tocontrol this yourself.

3. Put H_FIXED=0. on the SURF line. This turns off the convective heat flux from gas to surface and visverse. The heat flux to the solid is specified via EXTERNAL_FLUX (kW/m2) on the SURF line that isassigned to the solid surface. If you want to specify a particular convective heat flux to the solid surface,you can set ASSUMED_GAS_TEMPERATURE on the MISC line, along with a non-zero value of H_FIXEDon SURF in units of W/m2/K.

4. Turn off all the gas phase computations by setting SOLID_PHASE_ONLY=.TRUE. on the MISC line. Thiswill also speed up the computations significantly. If the gas phase computations are needed, you mayturn off combustion by creating a REAC line with only Y_O2_INFTY=0.01. This sets the backgroundoxygen mass fraction to 0.01, too low to support any burning.

5. Generate MATL lines, plus a single SURF line, as you normally would, except add EXTERNAL_FLUX tothe SURF line. This is simply a “virtual” source that heats the solid. Think of this as a perfect radiantpanel or cone calorimeter.

6. Assign the SURF_ID to a VENT that spans the bottom of the computational domain. Create OPEN ventson all other faces.

7. Finally, add solid phase output devices to the solid surface, like ’WALL TEMPERATURE’, ’NET HEAT

FLUX’, ’BURNING RATE’, ’GAUGE HEAT FLUX’, and ’WALL THICKNESS’ (assuming the solid is toburn away). Use these to track the condition of the solid as a function of time. In particular, make surethat the ’BURNING RATE’ is appropriate for the particular external heat flux applied. Make sure thatthe ’WALL TEMPERATURE’ is appropriate. Compare your results to measurements made in a bench-scale device, like the cone calorimeter. Keep in mind, however, that the calculation and the experimentare not necessarily perfectly matched. The calculation is designed to eliminate uncertainties related toconvection, combustion, and apparatus-specific phenomena.

Example Case: thermoplastic

The term “thermoplastic” is often used to describe materials that essentially heat up and gasify withoutleaving any solid residue. The term is used here merely to indicate the class of materials for which thepyrolysis can be modeled with a single reaction that converts solid to gaseous fuel.

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The purpose of this example is to demonstrate how the solid phase pyrolysis model works in FDS.Essentially, the gas phase calculation is shut off except for the imposition of a 50 kW/m2 “external” heatflux. The solid in this example is a slab of plastic, similar in composition to PMMA. The solid is describedby the following SURF line:

&SURF ID='PMMA SLAB'COLOR='BLACK'MATL_ID='PMMA'THICKNESS=0.025EXTERNAL_FLUX=50. / External Flux is ONLY for this simple demo exercise

The COLOR is meaningless except to distinguish it in Smokeview. The EXTERNAL_FLUX is a virtual heatsource that is only used for these types of diagnostic exercises. The properties of the material are conveyedvia the MATL line:

&MATL ID='PMMA'CONDUCTIVITY=0.21SPECIFIC_HEAT=1.5DENSITY=1190.N_REACTIONS=1NU_FUEL=1.HEAT_OF_REACTION=1500.HEAT_OF_COMBUSTION=25000.REFERENCE_TEMPERATURE=330. /

Notice that in addition to k, ρ and c, there is one reaction specified, the yield of which is 100 % fuel gas(NU_FUEL=1.). The phase change from solid to gas consumes energy at a rate of 1,500 kJ/kg. Although notrelevant in this example, the burning of these fuel gases would produce energy at a rate of 25,000 kJ/kg. Thereaction nominally takes place at 330 C, a necessary parameter for the pyrolysis model described above.

The plots shown in Fig. 8.6 contain the results of the FDS simulation along with what is referred to as“simple theory.” By this, we mean that if all of the external heat flux were used to raise the solid temperatureto 330 C and then convert the solid to fuel gas, the results would be as shown by the “simple theory.” Ofcourse, the simple theory neglects heat losses through the solid and losses via radiation from the surface.Because it neglects these losses, the simple theory should be regarded as producing an upper bound onthe burning rate. Note that the small discontinuities in the FDS burning rate and temperature curves arenumerical rather than physical.

Because there is no solid residue produced by the single reaction, the sample thickness will graduallydecrease to zero. Note that this does not necessarily mean that the solid obstruction should disappear entirelyfrom the computational domain, but rather that the fuel should be consumed. The parameter BURN_AWAYis used to indicate that the solid ought to be completely removed, but because in this simple test casethe sample is aligned with the external boundary of the computational domain, the parameter BURN_AWAYwould have no effect. The slab is originally 0.025 m thick, with a density of 1190 kg/m3. This means thereis 29.75 kg/m2 of fuel present, which if divided by the burning rate (about 0.018 kg/m2/s) yields a burningtime of about 1700 s. After this, the fuel is consumed.

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0 500 1000 1500 20000

100

200

300

400

500

Time (s)

Surf

ace

Tem

per

atur

e(

C)

Temperature (thermoplastic)

Reference TemperatureFDS (Surface Temperature)

0 500 1000 1500 20000

10

20

30

40

50

60

Time (s)

Hea

tFlu

x(k

W/m

2)

Heat Flux (thermoplastic)

External Heat FluxFDS (Net Heat Flux)

0 500 1000 1500 20000

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Time (s)

Mas

sLos

sR

ate

(kg/

m2/s

)

Mass Loss Rate (thermoplastic)

Burn Rate (Simple Theory)FDS (Burn Rate)

0 500 1000 1500 20000

0.005

0.01

0.015

0.02

0.025

0.03

Time (s)

Thi

ckne

ss(m

)

Sample Thickness (thermoplastic)

Thickness (Simple Theory)FDS (Thickness)

Figure 8.6: Output of thermoplastic test case.

Example Case: charring_solid

This example just uses the solid phase algorithm. Essentially, the gas phase is shut off except for theimposition of a 50 kW/m2 “external” heat flux. The reaction mechanism is fairly complicated, as it includesvarious solids like cellulose, char, and water. The sample itself is described via a single SURF line:

&SURF ID='SPRUCE'STRETCH_FACTOR = 1.CELL_SIZE_FACTOR = 0.5MATL_ID(1,1:3) = 'CELLULOSE','WATER','LIGNIN'MATL_MASS_FRACTION(1,1:3) = 0.70,0.1,0.20MATL_ID(2,1) = 'CASI'THICKNESS(1:2) = 0.01,0.01EXTERNAL_FLUX = 50. /

The sample consists of two layers. The first is a combination of cellulose, water and lignin. There are MATLlines for each. Both cellulose and water have reaction parameters. Cellulose undergoes an endothermicreaction to form an “active” solid, and water evaporates. Lignin does not change form, at least not in thismodel. The second layer of the sample is a non-reacting slab of calcium silicate board. Note the twoparameters on the SURF line called STRETCH_FACTOR and CELL_SIZE_FACTOR. The former tells FDSnot to stretch the solid phase nodes used to solve the heat conduction equation. The latter tells FDS to usecells half the size of what it would use by default, based on the thermal properties. The intent of these

78

two parameters is to increase the spatial resolution of the solid phase mesh to increase the accuracy of thecalculation. These kinds of parameters are typically not specified by the user, but a sensitivity analysis mightindicate that they ought to be, especially when the reaction sequence is complicated as it is here.

Upon conversion of the virgin cellulose to its “active” component, this latter solid material undergoestwo reactions that occur at two different temperatures. The following MATL line describes what is to happen:

&MATL ID = 'ACTIVE'EMISSIVITY = 1.0CONDUCTIVITY_RAMP = 'k_cell'SPECIFIC_HEAT = 2.3DENSITY = 400.N_REACTIONS = 2A(1:2) = 1.3E10, 3.23E14E(1:2) = 1.505E5, 1.965E5HEAT_OF_REACTION(1:2) = 418., 418.NU_RESIDUE(1:2) = 0.35, 0.0NU_FUEL(1:2) = 0.65, 1.0RESIDUE(1) = 'CHAR' /

Notice that the reaction parameters are arrays containing the appropriate information for each reaction,referred to by the numbers 1 and 2. The first reaction converts 35 % (by mass) of the “active” solid to char,and 65 % to fuel gases. The second reaction converts all the mass to fuel gases.

Figure 8.7 shows the surface temperature and burning rate of the sample under the 50 kW/m2 externalheat flux. The burning rate peaks at the start of the simulation, decreases throughout the burning phase, andthen increases again at the end due to presence of an external backing material. The initial peak is typical ofchar-forming solids.

0 100 200 300 400 500 6000

100

200

300

400

500

600

700

800

Time (s)

Tem

per

atur

e(

C)

Surface Temperature (charring solid)

0 100 200 300 400 500 6000

0.005

0.01

0.015

0.02

Time (s)

Mas

sLos

sR

ate

(kg/

m2/s

)

Burning Rate (charring solid)

Figure 8.7: Output of charring_solid test case.

Example Case: couch and room_fire

In residential fires, upholstered furniture makes up a significant fraction of the combustible load. A singlecouch can generate several megawatts of energy and sometimes lead to compartment flashover. Modeling acouch fire requires a simplification of its structure and materials. At the very least, we want the upholsteryto be described as fabric covering foam:

&MATL ID = 'FABRIC'

79

FYI = 'Properties completely fabricated'SPECIFIC_HEAT = 1.0CONDUCTIVITY = 0.1DENSITY = 100.0N_REACTIONS = 1NU_FUEL = 1.REFERENCE_TEMPERATURE = 350.HEAT_OF_REACTION = 3000.HEAT_OF_COMBUSTION = 15000. /

&MATL ID = 'FOAM'FYI = 'Properties completely fabricated'SPECIFIC_HEAT = 1.0CONDUCTIVITY = 0.05DENSITY = 40.0N_REACTIONS = 1NU_FUEL = 1.REFERENCE_TEMPERATURE = 350.HEAT_OF_REACTION = 1500.HEAT_OF_COMBUSTION = 30000. /

&SURF ID = 'UPHOLSTERY'FYI = 'Properties completely fabricated'COLOR = 'PURPLE'BURN_AWAY = .TRUE.MATL_ID(1:2,1) = 'FABRIC','FOAM'THICKNESS(1:2) = 0.002,0.1PART_ID = 'smoke' /

Both the fabric and the foam decompose into fuel gases via single-step reactions. The fuel gases from eachhave different composition and heats of combustion. FDS automatically adjusts the mass loss rate of each sothat the “effective” fuel gas is that specified by the user on the REAC line. The attribute BURN_AWAY forcesFDS to break up the couch into individual cell-sized blocks that will disappear from the calculation as soonas the fuel is exhausted. The surface is specified as consisting of two layers, with a thickness of 2 mm forthe FABRIC and 10 cm for the FOAM. The 10 cm is chosen to be the same as the mesh cell size.

The same couch model is included in a room-scale fire simulation, known as the room_fire test case.Figure 8.8 shows the fire after 5 and 10 minutes, respectively. Note that after 5 minutes, the couch is fully-involved, and after 10 minutes the room has flashed over. Only the reaction zone of the fire is shown; thesmoke is hidden so that you can see the fire progressing from the couch to the doorway at the right of thescene. This door is the only opening to the compartment, and after 10 minutes, the flames can be seenflowing out.

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Figure 8.8: Output of room_fire test case showing fire after 5 and 10 minutes, respectively.

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Example Case: cable_tray

A common combustible in industrial settings are power, control, and instrument cables. The cables maybe bundled in a variety of conduits; the most common of which is a ladder-like “tray.” From the point ofview of FDS, the pile of cables in a tray is a composite of a variety of plastics, insulators, and metal, usuallycopper. Here is one way to describe a tray of cables:

&MATL ID = 'PLASTIC'CONDUCTIVITY = 0.2SPECIFIC_HEAT = 1.5DENSITY = 1500.N_REACTIONS = 1HEAT_OF_REACTION = 3000.HEAT_OF_COMBUSTION = 25000.REFERENCE_TEMPERATURE = 400.NU_FUEL = 1.0 /

&MATL ID = 'COPPER'SPECIFIC_HEAT = 0.38CONDUCTIVITY = 387.DENSITY = 8940. /

&SURF ID = 'Loose Cable'COLOR = 'IVORY BLACK'MATL_ID(1,1:2) = 'PLASTIC','COPPER'MATL_MASS_FRACTION(1,1:2) = 0.4,0.6BACKING = 'EXPOSED'THICKNESS = 0.02 /

&OBST XB=-2.00, 2.00,-0.14, 0.14, 0.51, 0.55, SURF_ID='Loose Cable' /

&OBST XB=-2.00, 2.00,-0.15,-0.15, 0.50, 0.60, SURF_ID='SHEET METAL' / Tray Side&OBST XB=-2.00, 2.00, 0.15, 0.15, 0.50, 0.60, SURF_ID='SHEET METAL' / Tray Side&OBST XB=-1.95,-1.90,-0.15, 0.15, 0.50, 0.50, SURF_ID='SHEET METAL' / Rung&OBST XB=-1.60,-1.55,-0.15, 0.15, 0.50, 0.50, SURF_ID='SHEET METAL' / Rung...&OBST XB= 1.90, 1.95,-0.15, 0.15, 0.50, 0.50, SURF_ID='SHEET METAL' / Rung

The pile of cables is assumed to be a solid slab, 28 cm wide and 2 cm deep. The tray is slightly wider anddeeper, and because it is listed second in the input file, its surface properties take precedence wherever thecable slab and tray coincide. The mesh cells in this example are 5 cm on a side, but the heat transfer withinthe cable slab are governed by the 2 cm THICKNESS. The slab is 60 % copper, by mass. Note that we arenot assuming multiple layers in this example – the slab is a single layer composite of plastic and copper.The plastic burns at about 400 , but the copper remains. Thus, the cable does not “burn away.”

The point of this test case is merely to propose a simple model of flame spread along a tray of assortedcable. Detailed thermo-physical property data for industrial-grade cable is usually not available, and even ifit were, it would probably not improve upon the given model. The properties given in this example are almostcompletely fabricated. What is important here are the HEAT_OF_REACTION and REFERENCE_TEMPERATURE,obtained in most cases by a bench-scale measurement device like the cone calorimeter.

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Chapter 9

Ventilation

This chapter mainly describes how to specify velocity boundary conditions. For fire applications, this isessentially how a ventilation system is modeled.

9.1 Simple Vents, Fans and Heaters

For most applications, the ventilation system of a building is described in FDS using velocity boundaryconditions. For example, fresh air can be blown into, and smoke can be drawn from, a compartment byspecifying a velocity in the normal direction to a solid surface. However, there are various other facets ofvelocity boundary conditions that are described below.

9.1.1 Supply and Exhaust Vents

The easiest way to describe a supply or exhaust fan is to create a VENT positioned on a solid surface witha SURF_ID with some form of specified velocity or volume flow rate. The normal component of velocityis usually specified directly via the parameter VEL. If VEL is negative, the flow is directed into the compu-tational domain, i.e., a supply vent. If VEL is positive, the flow is drawn out of the domain, i.e., an exhaustvent. For example, the lines

&SURF ID='SUPPLY', VEL=-1.2, COLOR='BLUE' /&VENT XB=5.0,5.0,1.0,1.4,2.0,2.4, SURF_ID='SUPPLY' /

create a VENT that supplies air at a velocity of 1.2 m/s through an area of nominally 0.16 m2, dependingon the realignment of the VENT onto the FDS mesh. Regardless of the orientation of the plane x = 5,the flow will be directed into the room because of the sign of VEL. In this example the VENT may notbe exactly 0.16 m2 in area because it may not align exactly with the computational mesh. If this is thecase then VOLUME_FLUX can be prescribed instead of VEL. The units are m3/s. If the flow is entering thecomputational domain, VOLUME_FLUX should be a negative number, the same convention as for VEL. Notethat a SURF with a VOLUME_FLUX prescribed can be invoked by either a VENT or an OBST, but be aware thatin the latter case, the resulting velocity on the face or faces of the obstruction will be given by the specifiedVOLUME_FLUX divided by the area of that particular face. For example:

&SURF ID='SUPPLY', VOLUME_FLUX=-5.0, COLOR='GREEN' /&OBST XB=..., SURF_ID6='BRICK','SUPPLY','BRICK','BRICK','BRICK','BRICK' /

dictates that the forward x-facing surface of the obstruction is to have a velocity equal to 5 m3/s divided bythe area of the face (as approximated within FDS) flowing into the computational domain.

83

Note that either VEL or VOLUME_FLUX should be prescribed, not both. The choice depends on whether anexact velocity is desired at a given vent, or whether the given volume flux is desired.

9.1.2 Heaters

You can create a simple heating vent by changing the temperature of the incoming air

&SURF ID='BLOWER', VEL=-1.2, TMP_FRONT=50. /

The VENT with SURF_ID=’BLOWER’ would blow 50 C air at 1.2 m/s into the flow domain. Making VEL

positive would suck air out, in which case TMP_FRONT would not be necessary.

Note that if HRRPUA or solid phase reaction parameters are specified, no velocity should be prescribed. Thecombustible gases are ejected at a velocity computed by FDS.

9.1.3 Total Mass Flux

Most often, you specify a simple supply or exhaust vent by setting either a normal velocity or volume fluxat a solid surface. However, you may wish to control the mass flow rate (kg/s), as opposed to the volumeflow rate (m3/s), via the parameter MASS_FLUX_TOTAL. MASS_FLUX_TOTAL uses the same sign conventionas VEL above. In fact, the value entered for MASS_FLUX_TOTAL is converted internally into a velocityboundary condition whose value for an outflow is adjusted based on the local density.

9.1.4 Louvered Vents

Most real supply vents are covered with some sort of grill or louvers which act to redirect, or diffuse, theincoming air stream. It is possible to mimic this effect, to some extent, by prescribing both a normal andthe tangential components of the flow. The normal component is specified with VEL as described above.The tangential is prescribed via a pair of real numbers VEL_T representing the desired tangential velocitycomponents in the other two coordinate directions (x or y should precede y or z). For example, the line

&SURF ID='LOUVER', VEL=-1.2, VEL_T=0.5,-0.3 /

is a boundary condition for a louvered vent that pushes air into the space with a normal velocity of 1.2 m/sand a tangential velocity of 0.5 m/s in either the x or y direction and -0.3 m/s in either the y or z direction,depending on what the normal direction is.

In cases of limited mesh resolution, it may not be possible to describe a louvered vent or slot diffuserusing VEL_T because there may not be enough mesh cells spanning the opening. In these cases, you mightconsider simply specifying a flat plate obstruction in front of the VENT with an offset of one mesh cell. Theplate will simply redirect the air flow in all lateral directions.

9.1.5 Jet Fans

Fans do not have to be mounted on a solid wall, like a supply or an exhaust fan. If you just want to blow gasesin a particular direction, create a thin (zero cells thick) OBSTstruction and apply to it, via SURF_ID only,a SURF line that has the parameter POROUS=.TRUE. along with the other velocity parameters describedabove. This allows hot, smokey gases to pass through the obstruction, much like a free-standing fan. Theseobstructions are merely flat plates, by necessity. The velocity VEL associated with a POROUS surface is

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meant to represent the velocity in the positive or negative coordinate direction, as indicated by its sign. Thisis different than the convention used when the SURF is attached to a solid wall.

You may also want to construct a shroud around the fan using four flat plates arranged to form a shortpassageway that draws gases in one side and expels them out the other. The plate representing the fan itselfcan be positioned about halfway along the passage.

A SURF with POROUS=.TRUE. can only be applied to an OBSTruction. It has no meaning when applied toa VENT.

9.2 Species and Species Mass Flux Boundary Conditions

There are two species boundary conditions that can be specified (see Section 11.2 for details on inputtingand using species). These boundary conditions are MASS_FLUX(N) and MASS_FRACTION(N) where N

refers to a given species is via its place in the input file. For example, the second listed species is N=2.If a simple no-flux condition is desired at a solid wall, do not set anything. If the mass fraction of the Nthspecies is to be some value at a forced flow boundary (VEL or MASS_FLUX_TOTAL) set MASS_FRACTION(N)equal to the desired mass fraction on the appropriate SURF line. If the mass flux of the Nth species isdesired, set MASS_FLUX(N) instead of MASS_FRACTION(N). If MASS_FLUX(N) is set, no VEL shouldbe set. It is automatically calculated based on the mass flux. The inputs MASS_FLUX(N) (and typicallyMASS_FRACTION(N)) should only be used for inflow boundary conditions. MASS_FLUX(N) should bepositive with units of kg/m2/s.

Note that specifying MASS_FRACTION(N), sets the "ghost" cell values for the species mass fractions. Sincethe mass conservation equation is an advection-diffusion equation, if the specified velocity is small, then thediffusion term can dominate resulting in an unintended mass flux of species. To obtain a guaranteed massflux of a species, you should use MASS_FLUX(N)

9.3 Special Topic: Pressure Boundary Conditions

In some situations, it is more convenient to specify a pressure, rather than a velocity, at a boundary. Suppose,for example, that you are modeling the interior of a tunnel, and a wind is blowing at one of the portals thataffects the overall flow within the tunnel. If (and only if) the portal is defined using an OPEN vent, then thedynamic pressure at the boundary can be specified like this:

&VENT XB=..., SURF_ID='OPEN', DYNAMIC_PRESSURE=2.4, PRESSURE_RAMP='wind' /&RAMP ID='wind', T= 0.0, F=1.0 /&RAMP ID='wind', T=30.0, F=0.5 /

.

.

The use of a dynamic pressure boundary affects the FDS algorithm as follows. At OPEN boundaries, thehydrodynamic pressure (head) H is specified as

H = DYNAMIC_PRESSURE/ρ∞ + |u|2/2 (outgoing)

H = DYNAMIC_PRESSURE/ρ∞ (incoming) (9.1)

where ρ∞ is the ambient density and u is the most recent value of the velocity on the boundary. ThePRESSURE_RAMP allows you to alter the pressure as a function of time. Note that you do not need to ramp

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the pressure up or down starting at zero, like you do for various other ramps. The net effect of a positivedynamic pressure at an otherwise quiescent boundary is to drive a flow into the domain. However, a fire-driven flow of sufficient strength can push back against this incoming flow.

Example Case: pressure_boundary

The following lines, taken from the sample case, pressure_boundary, demonstrates how to do a time-dependent pressure boundary at the end of a tunnel. The tunnel is 10 m long, 1 m wide, 1 m tall with afire in the middle and a pressure boundary imposed on the right side. The left side (XMIN) is just an OPEN

boundary with no pressure specified. It is assumed to be at ambient pressure.

&VENT MB = 'XMIN' SURF_ID = 'OPEN' /&VENT MB = 'XMAX' SURF_ID = 'OPEN', DYNAMIC_PRESSURE=2.4, PRESSURE_RAMP='wind_ramp' /&RAMP ID='wind_ramp', T= 0., F= 1. /&RAMP ID='wind_ramp', T=15., F= 1. /&RAMP ID='wind_ramp', T=16., F=-1. /

Figure 9.1 shows two snapshots from Smokeview taken before and after the time when the positive pressureis imposed at the right portal of a tunnel. The fire leans to the left because of the preferential flow in thatdirection. It leans back to the right when the positive pressure is directed to become negative.

Figure 9.1: Snapshots from the sample case pressure_boundary showing a fire in a tunnel leaning left, thenright, due to a positive, then negative, pressure imposed at the right portal.

9.4 Special Topic: Fires and Flows in the Outdoors

Simulating a fire in the outdoors is not much different than a fire indoors, but there are a few issues thatneed to be addressed. First, the velocity of the wind profile at any exterior boundary will be a top hat(constant) by default, but the parameter PROFILE on the SURF line can yield other profiles. For exam-ple, PROFILE=’PARABOLIC’ produces a parabolic profile with VEL being the maximum velocity, and’ATMOSPHERIC’ produces a typical atmospheric wind profile of the form u = u0(z/z0)p. If an atmosphericprofile is prescribed, also prescribe Z0 for z0 and PLE for p. VEL specifies the reference velocity u0. Notethat z0 is not the ground, but rather some height where the wind speed is measured, like an elevated weatherstation. It is assumed that the ground is located at z = 0. To change this assumption, set GROUND_LEVEL onthe MISC line to be the appropriate value of z. Be careful not to apply an atmospheric velocity profile belowGROUND_LEVEL or FDS will stop with an error.

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Another useful parameter for outdoor simulations is the temperature lapse rate of the atmosphere. Typ-ically, in the first few hundred meters of the atmosphere, the temperature decreases several degrees Celsiusper kilometer. These few degrees are important when considering the rise of smoke since the temperature ofthe smoke decreases rapidly as it rises. The LAPSE_RATE of the atmosphere can be specified on the MISCline in units of C/m. A negative sign indicates that the temperature decreases with height. This need onlybe set for outdoor calculations where the height of the domain is tens or hundreds of meters. The defaultvalue of the LAPSE_RATE is 0 C/m.

By default, FDS assumes that the density and pressure decrease with height, regardless of the applicationor domain size. For most simulations, this effect is negligible, but it can be turned off completely by settingSTRATIFICATION=.FALSE. on the MISC line.

9.5 Tangential Velocity Boundary Conditions at Solid Surfaces

As a gas flows past a solid obstruction, it “sticks” to the surface and forms a boundary layer. Ideally, the gasvelocity at the surface is zero, and it increases rapidly through a boundary layer that is only a few millimetersthick to its “free-stream” value. In most practical simulations, it is not possible to resolve this boundary layerdirectly; thus, an empirical model is used to represent its effect on the overall flow field. For a DNS (DirectNumerical Simulation), this discussion is moot. The inner-wall “ghost” cell velocities are set so that thevelocity at the wall surface is zero. In other words, the velocity gradient at the wall is computed directlyfrom the resolved velocity vectors near the wall. For an LES (Large Eddy Simulation), the Werner-Wenglewall model is applied. See the FDS Technical Reference Guide [6] for details.

The parameter SLIP_FACTOR is no longer used to set the tangential velocity boundary condition, as ofFDS version 5.4.0. To force a solid boundary to have a free-slip condition, set FREE_SLIP=.TRUE. on theSURF line. To force a no-slip boundary condition, set NO_SLIP=.TRUE. on the SURF line. Otherwise, noparameters need be set – the appropriate wall model will be applied, depending on whether the calculationis a DNS or LES.

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9.6 Pressure-Related Effects: The ZONE Namelist Group (Table 15.28)

The basic FDS equation set assumes pressure to be composed of a “background” component, p(z, t), plusa perturbation, p(x, t). Most often, p is just the hydrostatic pressure, and p is the flow-induced pressurefield that FDS calculates at each time step. Originally (FDS v. 1-4), it was assumed that the backgroundpressure was the same throughout the computational domain. Because of this, it was only possible to createa single, sealed compartment whose walls conformed to the exterior of the computational domain. A fireor fan could increase (or decrease) the background pressure in this single compartment, and a leakage areacould be defined between the compartment and the ambient exterior. Flow through the “cracks” was simplya function of the background pressure via the usual empirical rules. This idea has been generalized startingin FDS 5. Now, you can specify any number of sealed compartments within the computational domain thatcan have their own “background” pressures, and these compartments, or “pressure zones,” can be connectedvia leakage and duct paths whose flow rates are tied to the pressure of the adjacent zones.

9.6.1 Specifying Pressure Zones

A pressure zone can by any region within the computational domain that is separated from the rest of thedomain, or the exterior, by solid obstructions. There is currently no algorithm within FDS to identify thesezones based solely on your specified obstructions. Consequently, it is necessary that you identify these zonesexplicitly in the input file. The basic syntax for a pressure ZONE is:

&ZONE XB=0.3,1.2,0.4,2.9,0.3,4.5 /

This means that the rectangular region, 0.3 < x < 1.2, 0.4 < y < 2.9, 0.3 < z < 4.5, is assumed to be withina sealed compartment. There can be multiple ZONEs declared. The indices of the zones, which are requiredfor the specification of leaks and fans, are determined simply by the order in which they are specified in theinput file. By default, the exterior of the computational domain is “Zone 0.” If there are no OPEN boundaries,the entire computational domain will be assumed to be “Zone 1.”

There are several restrictions to assigning pressure zones. First, the declared pressure zones must becompletely within a region of the domain that is bordered by solid obstructions. If the sealed region is notrectangular, FDS will extend the specified ZONE boundaries to conform to the non-rectangular domain. 1

It is possible to “break” pressure zones by removing obstructions between them.2 An example of how tobreak pressure zones is given in Section 9.3. Second, pressure zones can span multiple meshes, but it isrecommended that you check the pressure in each mesh to ensure consistency. Also, if the ZONE does spanmultiple meshes, make sure that the specified rectangular coordinates do so as well. This allows FDS todetermine the actual extent of the ZONE independently for each mesh.

Note that if you plan to have one zone open up to another via the removal of an obstruction, makesure that the coordinates of the two zones abut (i.e. touch) even if the one of the zones includes the solidobstruction that separates them. FDS recognizes that a zone boundary has been removed when two adjacentcells belonging to two different zones have no solid obstruction between them. It is recommended that youextend at least one of the zone boundaries into the solid obstruction separating the two zones. That way,when the obstruction is removed, the newly created gas phase cells will be assigned to one or the other zoneand it will become obvious that two adjacent gas phase cells are of two different zones, at which point thezones will merge and no longer have distinct background pressures.

1The extension of pressure zones to non-rectangular regions is a feature that started with FDS version 5.3.1.2The ability to open pressure zones became available in FDS starting with version 5.3.0. Prior versions prevented it by issuing

an error statement.

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9.6.2 Leaks

The volume flow, V , through a leak of area AL is given by

Vleak = AL sign(∆p)

√2|∆p|ρ∞

(9.2)

where ∆p is the pressure difference between the adjacent compartments (in units of Pa) and ρ∞ is the ambientdensity (in units of kg/m3). The discharge coefficient normally seen in this type of formula is assumed tobe 1. Leakage is inherently a subgrid-scale phenomenon because the leakage area is usually very small.In other words, it is not possible to define a leak directly on the numerical mesh. It is sometimes possibleto “lump” the leaks into a single mesh-resolvable hole, but this is problematic for two reasons. First, theleakage area rarely corresponds neatly to the area of a single mesh cell-sized hole. Second, the flow speedsthrough the hole can be large and cause numerical instabilities.

A better way to handle leakage is by exploiting pressure zones. A pressure zone is a user-specifiedvolume within the computational domain that is entirely surrounded by solid obstructions. For example, theinterior of a closed room can be, and should be, declared a pressure zone. Leakage from one compartmentto another is then designated on the input lines defining the individual pressure ZONEs:

&ZONE XB=0.3,1.2,0.4,2.9,0.3,4.5, LEAK_AREA(0)=0.0001 /&ZONE XB=2.3,5.8,1.4,2.9,6.8,9.7, LEAK_AREA(1)=0.0002, LEAK_AREA(0)=0.0003 /

The first line designates a region of the computational domain to be “Pressure Zone” 1. Note that the orderof the ZONE lines is important; that is, the order implicitly defines Zone 1, Zone 2, etc. Zone 0 is by defaultthe ambient pressure exterior. In this example, a leak exists between Zone 1 and the exterior Zone 0, and thearea of the leak is 0.0001 m2 (1 cm by 1 cm hole, for example). Zone 2 leaks to Zone 1 (and vis verse) witha leak area of 0.0002 m2. Zone 2 also leaks to the outside with an area of 0.0003 m2. Note that zones neednot be connected for a leak to occur. At least one of the obstructions that form the walls of Zone 1 must havethe attribute LEAK_PATH=1,0, meaning that the leak between Zones 1 and 0 is uniformly distributed oversolids defined with:

&SURF ID='whatever',..., LEAK_PATH=1,0 /

Likewise, the boundaries of Zone 1 and Zone 2 must include solids whose SURF properties include LEAK_PATH=1,2,but these solids need not form a boundary between the two zones. The SURFaces with the LEAK_PATH

attribute lump all of the leakage over these areas. The order of the two pressure zones designated byLEAK_PATH is unimportant.

Example Case: pressure_rise

This example tests several basic features of FDS. A narrow channel, 3 m long, 0.002 m wide, and 1 mtall, has air injected at a rate of 0.1 kg/m2/s over an area of 0.2 m by 0.002 m for 60 s, with a linearramp-up and ramp-down over 1 s. The total mass of air in the channel at the start is 0.00718 kg. Thetotal mass of air injected is 0.00244 kg. The domain is assumed two-dimensional, the walls are adiabatic,and STRATIFICATION is set to .FALSE. simply to remove the slight change in pressure and density withheight. The domain is divided into three meshes, each 1 m long and each with identical gridding. We expectthe pressure, temperature and density to rise during the 60 s injection period. Afterwards, the temperature,density, and pressure should remain constant, according to the equation of state. The figures below showthe results of this calculation. The density matches exactly showing that FDS is injecting the appropriateamount of mass. The steady state values of the pressure, density and temperature are consistent with theideal values.

89

0 100 200 300 400 500 60010

20

30

40

50

60

70

Time (s)

Tem

per

atur

e(

C)

Temperature (pressure rise)

Ideal (Temp)FDS (Temp 3)

0 100 200 300 400 500 6000

1

2

3

4

5

6x 10

4

Time (s)

Pre

ssur

e(P

a)

Pressure (pressure rise)

Ideal (Pres)FDS (Pres 3)

0 100 200 300 400 500 6001

1.1

1.2

1.3

1.4

1.5

1.6

Time (s)

Den

sity

(kg/

m3)

Density (pressure rise)

Ideal (Dens)FDS (Dens 3)

Figure 9.2: Output of pressure_rise test case.

Example Case: zone_break

In the example case called zone_break, three simple compartments are initially isolated from each other andfrom the ambient environment outside. Air is blown into compartment 1 at a constant rate for 5 s, increasingits pressure approximately 9500 Pa. At 10 s, compartment 1 is opened to compartment 2, decreasing theoverall pressure in the two compartments to roughly one-third the original value because the volume of thecombined pressure zone has been roughly tripled. At 15 s, the pressure is further decreased by opening adoor to compartment 3, and, finally, at 20 s the pressure returns to ambient following the opening of a doorto the outside. Figure 9.3 displays the pressure within each compartment.

Notice that the pressure within each compartment does not come to equilibrium instantaneously. Rather, arelaxation factor is applied by FDS to bring the zones into equilibrium over several seconds. This is done forseveral reasons. First, in reality doors and windows do not magically disappear as they do in FDS. It takesa finite amount of time to fully open them, and the slowing of the pressure increase/decrease is a simpleway to simulate the effect. Second, relatively large pressure differences between zones wreak havoc withflow solvers, especially ones like FDS that use a low Mach number approximation. To maintain numericalstability, FDS gradually brings the pressures into equilibrium. This second point ought to be seen as awarning:

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0 5 10 15 20 25 300

2000

4000

6000

8000

10000

Time (s)

Pre

ssur

e(P

a)

Pressure (zone break)

Ideal (Pres1)Ideal (Pres2)Ideal (Pres3)FDS (pres 1)FDS (pres 2)FDS (pres 3)

Figure 9.3: Output of zone_break test case.

Do not use FDS to study the sudden rupture of pressure vessels! Its low Mach number formulation doesnot allow for high speed, compressible effects that are very important in such analyses. The zone breakingfunctionality described in this example is only intended to be used for relatively small pressure differences(<0.1 atm) between compartments. Real buildings cannot withstand substantially larger pressures anyway.

9.6.3 Fan Curves

In Section 9.1 there is a discussion of velocity boundary conditions, in which a fan is modeled simply as asolid boundary that blows or sucks air, regardless of the surrounding pressure field. In reality, fans operatebased on the pressure drop across the duct or manifold in which they are installed. A very simple “fan curve”is given by:

Vfan = AductUmax sign(∆pmax−∆p)

√|∆p−∆pmax|

∆pmax(9.3)

The volume flow in the absence of a pressure difference is given by the area of the duct times the velocityof the air. Aduct is the area of the duct (m2), and Umax is the air velocity (m/s) in the absence of a pressuredifference. The pressure difference, ∆p = p1− p2, indicates the difference in pressure between the down-stream compartment, or “zone,” and the upstream. The subscript 1 indicates downstream and 2 indicatesupstream. The term, ∆pmax, is the maximum pressure difference the fan can operate upon, and it is assumedto be a positive number. Figure 9.4 displays a typical fan curve.

The velocity of the fan in the absence of a pressure difference, Umax, is specified via the parameterVEL on the appropriate SURF line. Alternatively, the volume flow rate, AductUmax, can be specified usingVOLUME_FLUX. Do not use both. These parameters were already introduced in Section 7.1. To simulatethe behavior of a real fan, a few extra parameters need to be specified. To set ∆pmax, the maximum op-erating over-pressure, add MAX_PRESSURE to the SURF line. Note that MAX_PRESSURE should always bepositive and in units of Pa. If the pressure difference (downstream minus upstream) exceeds the specifiedMAX_PRESSURE, then there will be a backflow in the duct.

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Figure 9.4: Fan curve corresponding to VOLUME_FLUX=10 and MAX_PRESSURE=500. Note that a volumeflux greater than 10 is brought about by a negative pressure difference; that is, when the downstream pressureis less than the upstream. Note also that when the pressure difference is greater than 500 Pa, the volumeflow becomes negative; that is, the flow reverses.

Note that the rules governing the sign of VEL or VOLUME_FLUX remain in force for fans that are subjectto pressure differences between compartments. Simply note that the sign of either VEL or VOLUME_FLUXdefines “upstream” and “downstream.” Thus, the zone into which air is blown in the absence of a pressuredifference is the downstream zone.

Example Case: fan_test

Here is an example how fans can be specified. In it, two simple compartments share a common wall. Bothcompartments are considered as separate “pressure zones.” Two fans are mounted in the Partition Wall,blowing in opposite directions. The relevant input lines are:

&SURF ID='BLOW LEFT', POROUS=.TRUE., VEL=-0.2, DUCT_PATH=1,2, MAX_PRESSURE=1000. /&SURF ID='BLOW RIGHT', POROUS=.TRUE., VEL= 0.4, DUCT_PATH=2,1, MAX_PRESSURE=1000. /

&ZONE XB=-3.0, 0.0,-1.0, 1.0, 0.0, 2.0 / Pressure Zone 1&ZONE XB= 0.0, 3.0,-1.0, 1.0, 0.0, 2.0 / Pressure Zone 2

&OBST XB= 0.0, 0.0,-1.0, 1.0, 0.0, 2.0 / Partition Wall

&HOLE XB=-0.1, 0.1,-0.1, 0.1, 0.4, 0.6 /&OBST XB= 0.0, 0.0,-0.1, 0.1, 0.4, 0.6, ..., SURF_ID='BLOW RIGHT', PERMIT_HOLE=.FALSE. /

&HOLE XB=-0.1, 0.1,-0.1, 0.1, 1.4, 1.6 /&OBST XB= 0.0, 0.0,-0.1, 0.1, 1.4, 1.6, ..., SURF_ID='BLOW LEFT', PERMIT_HOLE=.FALSE. /

The volume flow through the fans is given by the expression:

Vfan = AductUmax sign(∆pmax−∆p)

√|∆p−∆pmax|

∆pmax(9.4)

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where Aduct is the area of the duct (both are 0.04 m2), Umax is the air velocity (0.4 m/s from Zone 1 toZone 2 and 0.2 m/s from Zone 2 to Zone 1), and ∆pmax is the maximum pressure difference the fan canoperate upon (in this case both fans use 1000 Pa).

In steady state, the volume flow from compartment to compartment (or Zone to Zone) should be equaland opposite in sign.

(0.04 m2)(0.4 m/s)

√|p2− p1−1000 Pa|

1000 Pa= (0.04 m2)(0.2 m/s)

√|p1− p2−1000 Pa|

1000 Pa(9.5)

The solution is p2 = 300 Pa and p1 =−300 Pa (see Fig. 9.5). Note that the sign of the Volume Flow in FDShas to do with whether the flow is moving in the plus or minus coordinate direction. This convention canmake these types of calculations a bit tricky.

0 10 20 30 40 50 60−400

−300

−200

−100

0

100

200

300

400

Time (s)

Pre

ssur

e(P

a)

Pressure (fan test)

Ideal (pres1)Ideal (pres2)FDS (pres 1)FDS (pres 2)

0 10 20 30 40 50 60−0.02

−0.015

−0.01

−0.005

0

0.005

0.01

0.015

0.02

Time (s)

Vol

ume

Flo

w(m

3/s

)

Volume Flow (fan test)

Ideal (vflow1)Ideal (vflow2)FDS (vflow1)FDS (vflow2)

Figure 9.5: Pressure and volume flow for the fan_test.

Consider a few of the extra parameters. The attribute POROUS=.TRUE. allows hot, smokey gases topass through the obstructions that represent the fans. These obstructions are merely flat plates, by necessity.The velocity VEL associated with a POROUS surface is meant to represent the velocity in the positive ornegative coordinate direction, as indicated by its sign. This is different than the convention used when theSURF is assigned to a solid wall. The DUCT_PATH defines the pressure ZONEs downstream and upstream ofthe fan, respectively. The fan represented by the the SURF line ID=’BLOW LEFT’, for example, blows airinto ZONE 1 and draws air out of ZONE 2 in the absence of a pressure difference between the compartments.The negative value of VEL indicates that the fan blows in the negative coordinate direction, into ZONE 1 andout of ZONE 2, and this is consistent with the order of the ZONEs listed by DUCT_PATH. In short, DUCT_PATHand VEL must be coordinated.

The HOLEs in the Partition Wall serve only to carve out space for the obstructions that represent the fans.Note the obstructions have zero thickness, as required by the POROUS surface. The attribute PERMIT_HOLE=.FALSE.tells FDS not to reject the obstructions because they are embedded within the Partition Wall.

Example Cases: leak_test and leak_test_2

In the following examples, both leaks and fans are demonstrated. A simple compartment (3.6 m by 2.4 m by2.4 m) has a small fan at one end and one leak under the door at the other end. It is assumed for this examplethat the compartment is contained within a larger compartment that is perfectly sealed. The fan draws air

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into the compartment from the plenum space, increasing the pressure inside and decreasing it outside. Asteady state is achieved when the volume flow into and out of the compartment falls into balance.

The volume flow rate of the fan is given by the “fan curve”

Vfan = AductUmax sign(∆pmax−∆p)

√|∆p−∆pmax|

∆pmax(9.6)

where ∆p is the difference in pressure and Aduct = 0.16 m2, Umax = 0.1 m/s, and ∆pmax = 1000 Pa. Thevolume flow due to the leak is given by:

Vleak = Aleak

√2∆pρ∞

(9.7)

where Aleak = 0.0001 m2 and ρ∞ = 1.2 kg/m3. After 5 min the pressure difference is 938.2 Pa. The theo-retical value, obtained by equating the fan and leak volume flow rates and solving for ∆p, is 938.9 Pa. Theslight difference is due to the fact that the solid boundaries within the interior of the computational domainadmit a slight volume flux related to details of the numerical solver.

Just for fun, we add another leak to the compartment, only this time the leak is to the exterior of theentire computational domain, an infinite void at ambient pressure. Now the fan flow rate ought to balancethe sum of the flow rates from the two leaks. After 5 min, the pressure difference is 935.2 Pa. The two casesare summarized in Fig. 9.6.

0 50 100 150 200 250 300−1000

−500

0

500

1000

Time (s)

Pre

ssur

e(P

a)

Pressure (leak test)

Ideal (pres1)

Ideal (pres2)

FDS (pres 1)

FDS (pres 2)

0 50 100 150 200 250 300−1000

−500

0

500

1000

Time (s)

Pre

ssur

e(P

a)

Pressure (leak test 2)

Ideal (pres1)Ideal (pres2)FDS (pres 1)FDS (pres 2)

Figure 9.6: Output of the leak test cases.

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Chapter 10

User-Specified Functions

Many of the parameters specified in the input file are fixed constants. However, there are several parametersthat may vary in time, temperature, or space. These functions can be complex, thus you have to have someway to convey them. The namelist group RAMP and TABL, as it names imply, allow you to control thebehavior of selected parameters. RAMP allows you to specify a function with one independent variable (suchas time) is mapped to one dependent variable (such as velocity). TABL allows for the specification of amapping from multiple independent variables (such as a solid angle) to multiple dependent variables (suchas a sprinkler flow rate and droplet speed).

10.1 Time-Dependent Functions

At the start of any calculation, the temperature is ambient everywhere, the flow velocity is zero everywhere,nothing is burning, and the mass fractions of all species are uniform. When the calculation starts temper-atures, velocities, burning rates, etc., are ramped-up from their starting values because nothing can happeninstantaneously. By default, everything is ramped-up to their prescribed values in roughly 1 s. However,control the rate at which things turn on, or turn off, by specifying time histories for the boundary condi-tions that are listed on a given SURF line. The above boundary conditions can be made time-dependentusing either prescribed functions or user-defined functions. The parameters TAU_Q, TAU_T, and TAU_V in-dicate that the heat release rate (HRRPUA); surface temperature (TMP_FRONT); and/or normal velocity (VEL,VOLUME_FLUX), or MASS_FLUX_TOTAL are to ramp up to their prescribed values in TAU seconds and re-main there. If TAU_Q is positive, then the heat release rate ramps up like tanh(t/τ). If negative, then theHRR ramps up like (t/τ)2. If the fire ramps up following a t2 curve, it remains constant after TAU_Q sec-onds. These rules apply to TAU_T and TAU_V as well. The default value for all TAUs is 1 s. If somethingother than a tanh or t2 ramp up is desired, then a user-defined burning history must be input. To do this,set RAMP_Q, RAMP_T or RAMP_V equal to a character string designating the ramp function to use for thatparticular surface type, then somewhere in the input file generate lines of the form:

&RAMP ID='rampname1', T= 0.0, F=0.0 /&RAMP ID='rampname1', T= 5.0, F=0.5 /&RAMP ID='rampname1', T=10.0, F=0.7 /

.

.

.&RAMP ID='rampname2', T= 0.0, F=0.0 /&RAMP ID='rampname2', T=10.0, F=0.3 /&RAMP ID='rampname2', T=20.0, F=0.8 /

.

95

.

.

Here, T is the time, and F indicates the fraction of the heat release rate, wall temperature, velocity, massfraction, etc., to apply. Linear interpolation is used to fill in intermediate time points. Note that each setof RAMP lines must have a unique ID and that the lines must be listed with monotonically increasing T.Note also that the TAUs and the RAMPs are mutually exclusive. For a given surface quantity, both cannot beprescribed.

As an example, the simple blowing vent from above can be controlled via the lines:

&SURF ID='BLOWER',VEL=-1.2,TMP_FRONT=50., RAMP_V='BLOWER RAMP', RAMP_T='HEATER RAMP' /&RAMP ID='BLOWER RAMP', T= 0.0, F=0.0 /&RAMP ID='BLOWER RAMP', T=10.0, F=1.0 /&RAMP ID='BLOWER RAMP', T=80.0, F=1.0 /&RAMP ID='BLOWER RAMP', T=90.0, F=0.0 /&RAMP ID='HEATER RAMP', T= 0.0, F=0.0 /&RAMP ID='HEATER RAMP', T=20.0, F=1.0 /&RAMP ID='HEATER RAMP', T=30.0, F=1.0 /&RAMP ID='HEATER RAMP', T=40.0, F=0.0 /

Now the temperature and velocity of the incoming air stream would follow the same ramp functions. Notethat the temperature and velocity can be independently controlled by assigning different RAMPs to RAMP_T

and RAMP_V, respectively.Use TAU_T or RAMP_T to control the ramp-ups for surface temperature. The surface temperature will

be computed asTW (t) = TAMB+ f (t)(TMP_FRONT−TAMB) (10.1)

where TW (t) is the surface temperature to be applied, f (t) is the result of evaluating the RAMP_T at time t,TAMB is the input specified on the MISC line, and TMP_FRONT is the input specified on the SURF line thatthe RAMP_T is for.

Use TAU_MF(N) or RAMP_MF(N) to control the ramp-ups for either the mass fraction or mass flux ofspecies N. The mass fraction of species N at the surface is given by

YN(t) = YN(0)+ f (t)(YN−YN(0))

where YN(0) is the ambient mass fraction of species N (MASS_FRACTION_0 in the Nth SPEC namelistline is used to prescribe YN(0)), YN is the desired mass fraction to which the function f (t) is ramping(MASS_FRACTION(N) specified in the SURF line is used to prescribe YN). The function f (t) is either atanh, t2, or user-defined function. For a user-defined function, indicate the name of the ramp function withRAMP_MF(N), a character string.

10.2 Temperature-Dependent Functions

Thermal properties like conductivity and specific heat can vary significantly with temperature. In such cases,use the RAMP function like this:

&MATL ID = 'STEEL'FYI = 'A242 Steel'SPECIFIC_HEAT_RAMP = 'c_steel'CONDUCTIVITY_RAMP = 'k_steel'DENSITY = 7850. /

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&RAMP ID='c_steel', T= 20., F=0.45 /&RAMP ID='c_steel', T=377., F=0.60 /&RAMP ID='c_steel', T=677., F=0.85 /

&RAMP ID='k_steel', T= 20., F=48. /&RAMP ID='k_steel', T=677., F=30. /

Note that here (as opposed to time ramps) the parameter F is the actual physical quantity, not just a fraction ofsome other quantity. Thus, if CONDUCTIVITY_RAMP is used, there should be no value of CONDUCTIVITYgiven. Note also that for values of temperature, T, below and above the given range, FDS will assume aconstant value equal to the first or last F specified.

10.3 Tabular Functions

Some input quantities, such as a sprinkler spray pattern, vary multi-dimensionally. In such cases, use theTABL namelist group. The format of the TABL lines is application-specific, but in general look like this:

&TABL ID='TABLE1', TABLE_DATA=40,50, 85, 95,10,0.5 /&TABL ID='TABLE1', TABLE_DATA=40,50,185,195,10,0.5 /

A detailed description of the various table entries is given in the sections that describe quantities that usesuch tables. Currently, only sprinklers and nozzles use this group of parameters to define a complex spraypattern.

Note that each set of TABL lines must have a unique ID. Specific requirements on ordering the lines willdepend upon the type of TABL and those requirements are provided in the appropriate section in this guide.

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Chapter 11

Combustion and Radiation

A common source of confusion in FDS is the distinction between gas phase combustion and solid phasepyrolysis. The former refers to the reaction of fuel vapor and oxygen; the latter the generation of fuel vaporat a solid or liquid surface. Whereas there can be many types of combustibles in an FDS fire simulation,there can only be one gaseous fuel. The reason is cost. It is expensive to solve transport equations formultiple gaseous fuels. Consequently, the burning rates of solids and liquids are automatically adjusted byFDS to account for the difference in the heats of combustion of the various combustibles. In effect, youspecify a single gas phase reaction as a surrogate for all the potential fuel sources.

The gas phase reaction can be described in two ways. By default, a so-called mixture fraction model isused to account for the evolution of the fuel gas from its surface of origin through the combustion process.The alternative is what is referred to as the finite-rate approach, where all of the individual gas speciesinvolved in the combustion process are defined and tracked individually, and the combustion process ismodeled as one or more finite-rate reactions of these species. This is a costlier and more complicatedapproach than the mixture fraction model. This chapter describes both methods, with an emphasis on themore commonly used mixture fraction model.

11.1 Mixture Fraction Combustion: The REAC Namelist Group

There are two ways of modeling a fire. The first is to specify explicitly a Heat Release Rate Per Unit Area,HRRPUA, on a SURF line and then apply the SURF_ID to an obstruction or vent. This essentially creates a gasburner whose fuel flow rate you explicitly control. The other way to model a fire is to specify solid phasethermal and pyrolysis properties on one or more MATL lines, assemble these materials via a SURF line, andthen apply this boundary condition to obstructions or vents. In this case the burning rate of the fuel dependson the net heat feedback to the surface. In both cases, however, the mixture fraction combustion model isused by default. In fact, the mere presence of these parameters automatically invokes the mixture fractionmodel1. Do not specify explicitly gas species like oxygen if you have also specified the heat release rate perunit area, HRRPUA, or solid phase reaction rates.

11.1.1 Basics

The stoichiometry of the gas phase combustion reaction or reactions is specified using the REAC namelistgroup. For the default mixture fraction model, only a single REAC line is needed. If the REAC line is not foundin the input file, propane will be used as the surrogate fuel, and all burning rates will be adjusted accordingly.

1There is an exception to this rule. It is possible to use the finite-rate combustion model in conjuction with a pyrolysis modelthat generates the specified gaseous fuel species.

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If you only specify the fire’s heat release rate with HRRPUA, then the reaction parameters may not requireadjusting, and no REAC line need be added to the input file. However, if you know something about thepredominant fuel gas, you might want to consider specifying, at the very least, the basic stoichiometry viathe REAC line.

Using the mixture fraction model, each reaction is assumed to be of the form:

CxHyOzNvOtherw +νO2 O2→ νCO2 CO2 +νH2O H2O+νCO CO+νsoot Soot+νN2 N2 +νH2 H2 +νOther Other(11.1)

You need only specify the chemical formula of the fuel along with the yields of CO, soot, and H2, and theamount of hydrogen in the soot, H f rac. For completeness you can specify the N2 content of the fuel and thepresence of other species. FDS will use that information internally to determine the amount of combustionproducts that are formed:

νO2 = νCO2 +νCO

2+

νH2O

2− z

2νCO2 = x−νCO− (1−Hfrac)νsoot

νH2O =y2− Hfrac

2νsoot−νH2

νCO =Wf

WCOyCO

νH2 =Wf

WH2

yH2

νsoot =Wf

Wsys

νN2 =v2

νother = w

Ws = Hfrac WH +(1−Hfrac)WC

The following parameters may be prescribed on the REAC line. Note that the various YIELDs are for well-ventilated, post-flame conditions. There are options to predict various species yields in under-ventilated firescenarios, but these special models still require the post-flame yields for CO, soot and any other specieslisted below.

FUEL A character string that identifies fuel species for the reaction. When using the mixture fraction,specifying a fuel will cause FDS to use the thermophyscial properties for that fuel when computingquantites such as specific heat or viscosity. This parameter is independent of the inputs for the fuelchemistry, i.e. C, H, O, N, OTHER. Table 11.1 provides a listing of the available species. By default,FDS uses the gas thermophysical properties of ETHYLENE for the fuel.

ID A character string that identifies the reaction. Normally, this label is not used by FDS, but it is useful tolabel the REAC line to more easily identify the fuel species.

C, H, O, N, OTHER The fuel chemical formula. All numbers are positive. (Mixture Fraction only, de-fault values are those of propane)

MW_OTHER Average molecular weight for OTHER (g/mol). (Mixture Fraction only, default is the molecularweight of N2, 28 g/mol)

Y_O2_INFTY Ambient mass fraction of oxygen (Mixture Fraction only, default 0.232428)

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Y_F_INLET Mass fraction of fuel in fuel stream (Mixture Fraction only, default 1.0)

SOOT_YIELD The fraction of fuel mass converted into smoke particulate, ys. Note that this parameter doesnot apply to the processes of soot growth and oxidation, but rather to the net production of the smokeparticulate from the fire. (Mixture Fraction only, default 0.01)

SOOT_H_FRACTION The fraction of the atoms in the soot that are hydrogen. (Mixture Fraction only, default0.1)

CO_YIELD The fraction of fuel mass converted into carbon monoxide, yCO. (Mixture Fraction only, default0.0)

H2_YIELD The fraction of fuel mass converted into hydrogen, yH2 . (Mixture Fraction only, default 0.0)

HEAT_OF_COMBUSTION ∆H (kJ/kg). The amount of energy released per unit mass of fuel consumed. Notethat if the heat of combustion is not specified, it is assumed to be

∆H ≈ νO2 WO2

ν f WfEPUMO2 kJ/kg (11.2)

EPUMO2 The amount of energy released per unit mass of oxygen consumed. (kJ/kg) Default is 13,100 kJ/kg.Note that if both EPUMO2 and HEAT_OF_COMBUSTION are specified that FDS will ignore the value forEPUMO2.

IDEAL Logical value indicating whether or not the EPUMO2 or HEAT_OF_COMBUSTION values representvalues for complete combustion (.TRUE.) or for incomplete combustion (.FALSE.), i.e. the valuesaccount for the specified yCO, yH2 , and ys. If IDEAL=.TRUE., then FDS will internally adjust ∆H toaccount for products of incomplete combustion. The default value is .FALSE.

A few sample REAC lines are given here. The values are for demonstration only.

&REAC ID = 'METHANE'C = 1.H = 4. /

&REAC ID = 'PROPANE'SOOT_YIELD = 0.01C = 3.H = 8.HEAT_OF_COMBUSTION = 46460.IDEAL = .TRUE. /

&REAC ID = 'PROPANE'SOOT_YIELD = 0.01C = 3.H = 8.HEAT_OF_COMBUSTION = 46124.IDEAL = .FALSE. /

&REAC ID = 'ACRYLONITRILE'C = 3.H = 3.N = 1.HEAT_OF_COMBUSTION = 24500.IDEAL = .TRUE. /

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&REAC ID = 'CARBON DISULFIDE'C = 1.Other = 2.MW_OTHER = 32.HEAT_OF_COMBUSTION = 13600.IDEAL = .TRUE. /

11.1.2 Special Topic: Heat of Combustion

By default, the Energy Per Unit Mass of Oxygen, EPUMO2, is used to compute the heat of combustionaccording to Eq. (11.2). Specifying the HEAT_OF_COMBUSTION will override this computation. However, ifheats of reaction have been specified on the MATL line and the heat of combustion of the material differs fromthat specified by the governing gas phase reaction, then add a HEAT_OF_COMBUSTION (kJ/kg) to the MATLline. With the mixture fraction combustion model, it is assumed that there is only one fuel. However, in arealistic fire scenario, there may be many fuel gases generated by the various burning objects in the building.Specify the stoichiometry of the predominant reaction via the REAC namelist group. If the stoichiometry ofthe burning material differs from the global reaction, the HEAT_OF_COMBUSTION is used to ensure that anequivalent amount of fuel is injected into the flow domain from the burning object.

11.1.3 Special Topic: Flame Extinction

Modeling suppression of a fire due to the introduction of a suppression agent like CO2 or water mist, or dueto the exhaustion of oxygen within a compartment is challenging because the relevant physical mechanismsoccur at length scales smaller than a single mesh cell. Flames are extinguished due to lowered temperaturesand dilution of the oxygen supply. A simple suppression algorithm has been implemented in FDS thatattempts to gauge whether or not a flame is viable at the fuel-oxygen interface. The Technical ReferenceGuide [1] contains more details about how the mechanism works. The only parameters you can control arethe Limiting Oxygen Index, X_O2_LL, and the CRITICAL_FLAME_TEMPERATURE. Both are set on the REACline. The default values are 0.15 (volume fraction) and 1427 C, respectively. To eliminate any gas phasesuppression, set X_O2_LL to 0, or turn off suppression completely by setting SUPPRESSION=.FALSE. onthe MISC line. This latter approach saves on computing time because it prevents FDS from entering thesuppression algorithm altogether.

Example Case: door_crack

This example uses the same simple compartment that was used to test leakage and fan curves. Now, weadd a small (160 kW) fire, with the same fan and leak under the door. The compartment now opens to theatmosphere, not a sealed plenum. We expect a rapid pressure rise in the compartment due to the effect ofthe fire and the fan. Initially, the pressure rises due to the heat from the fire and the fan blowing air into thecompartment:

d p1

dt≈ (γ−1)

QV

+ γ pVV≈ 3200 Pa/s≈ 0.03 atm/s (11.3)

where γ ≈ 1.4, Q = 160,000 W, V = 20.7 m3, and V = 0.016 m3/s. However, as heat is lost to the walls,and as the leak begins to relieve the pressure increase, the pressure rise slows, and then reverses at about0.25 atm. In roughly 150 s, the fire is self-extinguished due to lack of oxygen. As the pressure rise decreasesback to zero, and actually goes below zero, the fan, and the leak under the door, increase the oxygen supply,at least near these openings, and the fuel-rich gases in the compartment continue to burn.

While this case has a number of interesting physical effects, and it verifies several features of FDS, it isvery important to note the following:

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0 50 100 150 200 250 300−1

−0.5

0

0.5

1

1.5

2

2.5

3x 10

4

Time (s)

Pre

ssur

e(P

a)

Pressure (door crack)

0 50 100 150 200 250 3000

50

100

150

200

Time (s)

Hea

tR

elea

seR

ate

(kW

)

HRR (door crack)

Figure 11.1: Output of door_crack test case.

• Although there is smoke seen flowing backwards out the fan duct, in reality there would have been muchmore. Most conventionally built structures will not withstand over-pressures of 0.25 atm without somesort of relief. The fan and the crack under the door obey simple formulae based on pressure differences,but these assumptions have limits.

• It is likely that the fire in this scenario would indeed extinguish itself as the oxygen volume fractiondecreased below about 15 %. But, its re-ignition at the door crack and fan grill would depend on thepresence of a spark or hot spot of some sort. FDS continues to flow fuel into the compartment past thepoint of local extinction, but the compartment cools. The default combustion algorithm in FDS assumesthat in every grid cell there is a “virtual spark plug” that initiates combustion if fuel and oxygen arepresent.

11.1.4 Special Topic: CO Production

An algorithm has been implemented that computes the combustion as a two step reaction that predicts theformation and destruction of CO. The Technical Reference Guide [1] contains more details about how themechanism works. This algorithm is used when CO_PRODUCTION is set to .TRUE. on the MISC line. Eventhough the algorithm predicts CO formation and its eventual oxidation at elevated temperature, it cannotpredict the post-flame yield of CO. For example, within a flashed over compartment, the algorithm predictsthe elevated CO levels, but it cannot predict the CO concentration of the exhaust gases that exit the flamingregion. Thus, even if using this model, you must specify the CO_YIELD that is expected of a well-ventilatedfire.

Note that when active, this algorithm requires the use of three parameters for the mixture fraction comparedto the default two and will therefore increase run times and memory usage accordingly. If the simulationyou are performing will not result in an under-ventilated fire, then there will be of little if any benefit toenabling the CO production algorithm.

11.1.5 Special Topic: Turbulent Combustion

Unless you are performing a Direct Numerical Simulation (DNS), the reaction rate of fuel and oxygen is notbased on the diffusion of fuel and oxygen at a well-resolved flame sheet. Instead, semi-empirical rules are

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invoked by FDS to determine the rate of mixing of fuel and oxygen within a given mesh cell at a given timestep. This section provides a brief explanation of these rules and the parameters that control them.

In an LES simulation, the heat release rate is computed as

q′′′ = min(

q′′′max ,min(ρYF,sρYO2)

τ∆H)

; s =WF

νO2WO2

(11.4)

Here, τ is a mixing time scale [8] given by

τ =C (δxδyδz)

23

DLES

(11.5)

The coefficient, C, is taken as 0.1 in FDS by default, based on comparisons to various flame height correla-tions. It is given by C_EDC on the REAC line. If you do not want to use this model, setEDDY_DISSIPATION=.FALSE.

on the REAC line, in which case τ becomes the time step, δt. There is an additional bound on the local heatrelease rate per unit volume, based on an empirical estimate of the average volumetric heat release rate of afire. Orloff and De Ris [9] suggest a value of 1200 kW/m3 for the entire fire. FDS uses by default a value of

q′′′max =q′′max

δx+ q′′′max kW/m3 (11.6)

as a local upper bound. The term, q′′max, is the maximum heat release rate per unit area of flame sheet. It isspecified via HRRPUA_SHEET on the REAC line. It is 0 kW/m2 by default for LES; 200 for DNS. The term,q′′′max, is the maximum heat release rate per unit volume. It is specified via HRRPUV_AVERAGE on the REACline. It is 2500 kW/m3 by default for LES; 0 for DNS. Further discussion is found in the FDS TechnicalReference Guide, Volume 1.

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11.2 Extra Gas Species: The SPEC Namelist Group

Normally when you specify a fire via either HRRPUA on the SURF line or reaction parameters on the MATLline, the mixture fraction combustion model is applied. A a set of two or three scalar variables, Zi, representthe state of the combustion process from pure fuel (∑Zi = 1) to pure air (∑Zi = 0). The major reactantsand products of combustion – fuel, O2, CO2, H2O, N2, CO and soot – are all pre-tabulated functions of themixture fraction, Z. In other words, the values of Zi in any given mesh cell determines the mass fractionof all the gases listed. The fuel chemistry listed under the REAC namelist group is used to generate thetable associating the mass fractions with Zi. You need not, and should not, explicitly list the reactants andproducts of combustion.

Suppose however that gases are introduced into the domain that are neither reactants nor products ofcombustion. This gas can be tracked separately from the mixture fraction via an additional scalar transportequation2. In fact, there does not need to be any fire at all – FDS can be used to transport a mixture ofnon-reacting ideal gases.

11.2.1 Basics

The namelist group SPEC is used to specify each additional gas species. Each SPEC line should include atthe very least the name of the species via a character string called (ID). Next, if the ambient (initial) massfraction of the gas is something other than 0, then the parameter MASS_FRACTION_0 is used to specify it.Several gases that can be included in a calculation are listed in Table 11.1. Here is an example:

&SPEC ID='ARGON',MASS_FRACTION_0=0.1,MW=40. /

Once the extra species has been declared, you introduce it at surfaces via the parameters MASS_FRACTION(N)or MASS_FLUX(N). The index N refers to the order in which the species is listed in the input file. Followingis a very simple example of how a gas can be introduced into the simulation.

Sample Case: gas_filling

Consider the short input file:

&HEAD CHID='gas_filling', TITLE='Fill an Empty Room with Hydrogen' /&MESH IJK=32,32,15, XB=-3.2,3.2,-3.2,3.2,0.0,3.0 /&TIME T_END=300.0 /&SPEC ID='HYDROGEN' /&SURF ID='LEAK', MASS_FLUX(1)=0.01667, RAMP_MF(1)='leak_ramp' /&RAMP ID='leak_ramp', T= 0., F=0.0 /&RAMP ID='leak_ramp', T= 1., F=1.0 /&RAMP ID='leak_ramp', T=180., F=1.0 /&RAMP ID='leak_ramp', T=181., F=0.0 /&VENT XB=-0.6,0.4,-0.6,0.4,0.0,0.0, SURF_ID='LEAK', COLOR='RED' /&DUMP MASS_FILE=.TRUE. /&SLCF PBY=0.0, QUANTITY='HYDROGEN' /&TAIL /

The case is nothing more than hydrogen gas filling a box. The gas is injected through a 1 m by 1 m vent at arate of 0.01667 kg/m2/s and shut off after 3 m. The total mass of hydrogen at that point ought to be 3 kg (see

2 Often an extra gas introduced into a calculation is the same as a product of combustion, like water vapor from a sprinkler orcarbon dioxide from an extinguisher. These gases are tracked separately, thus water vapor generated by the combustion is tracked viathe mixture fraction variable and water vapor generated by evaporating sprinkler droplets is tracked via its own transport equation.In the case of sprinklers, do not specify ’WATER VAPOR’ as an extra species – it is done automatically.

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Fig. 11.2). Notice that no properties were needed for the HYDROGEN because it is a species whose propertieshave already been programmed into FDS. The background species in this case is assumed to be air. Themass flow rate of the hydrogen is controlled via the ramping parameter RAMP_MF(1), and the index 1 refersto the first, and only, gas species that is specified in the input file. The parameter MASS_FILE=.TRUE.instructs FDS to produce an output file that contains a time history of the hydrogen mass.

0 50 100 150 200 250 3000

0.5

1

1.5

2

2.5

3

3.5

4

Time (s)

Hyd

roge

nM

ass

(kg)

Hydrogen Mass (gas filling)

Figure 11.2: Hydrogen mass vs. time for gas_filling test case.

11.2.2 Special Topic: Gas Species Properties

Gases whose properties are hardwired in FDS are listed in Table 11.1. The physical properties of thesegases are known and do not need to be specified. However, if a desired gas is not included in Table 11.1, itsmolecular weight MW must be specified on the SPEC line in units of g/mol. In addition, if a DNS calculationis being performed, either the Lennard-Jones potential parameters σ (SIGMALJ) and ε/k (EPSILONKLJ)should be specified; or the VISCOSITY (kg/m/s), CONDUCTIVITY (W/m/K), and DIFFUSIVITY (m2/s)between the given species and the background species should be specified.

If the simulation does not involve the mixture fraction model – either because no combustion is desiredor if a finite rate reaction(s) is being specified (see Section 11.2.4) – you can specify that the background gasspecies be something other than air. For a gas mixture comprised of n species, FDS only solves transportequations for n− 1 because it also solves an equation for total mass conservation. To set the properties ofthe implicitly defined BACKGROUND_SPECIES, use the MISC line. If this species is not listed in Table 11.1,specify its molecular weight, MW, and (optionally) its VISCOSITY; CONDUCTIVITY; and SPECIFIC_HEAT,SPECIFIC_ENTHALPY, and REFERENCE_TEMPERATURE. If either SPECIFIC_HEAT or ENTHALPY is spec-ified, then both must be specified. The REFERENCE_TEMPERATURE is the temperature that corresponds tothe specified value of SPECIFIC_ENTHALPY. In the absence of any of these parameters, the appropriatevalues of ’AIR’ are assumed. If the species listed in Table 11.1, is indicated as a liquid, that means thatliquid thermophysical properties do not need to be given for those species.

Recognized species that are emissive will been defined as ABSORBING and radiative absorption for thosespecies will be computed. The keyword ABSORBING can be specified on the SPEC line as well. If .TRUE.and the species is not in the recognized list, then it will be assumed to be a fuel when invoking RADCAL tocompute its absorptivity.

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Table 11.1: Optional Gas Species [10]

Species Mol. Wgt. σ ε/k Liquid(g/mol) (Å) (K)

AIR 29 3.711 78.6ARGON 40 3.42 124.0CARBON DIOXIDE 44 3.941 195.2CARBON MONOXIDE 28 3.690 91.7ETHANOL 46 4.530 362.6 YETHYLENE 28 4.163 224.7HELIUM 4 2.551 10.22HYDROGEN 2 2.827 59.7HYDROGEN BROMIDE 81 3.353 449.0HYDROGEN CHLORIDE 36 3.339 344.7HYDROGEN CYANIDE 26 3.63 569.1HYDROGEN FLOURIDE 20 3.148 330.0METHANE 16 3.758 148.6METHANOL 32 3.626 481.8 YN-HEXANE 86 4.524 199.41 YN-HEPTANE 100 4.701 205.75 YN-OCTANE 114 4.892 231.16N-DECANE 142 5.233 226.46NITROGEN 28 3.798 71.4OXYGEN 32 3.467 106.7PROPANE 44 5.118 237.1TOLUENE 92 5.698 480.0WATER VAPOR 18 2.641 809.1 Y

11.2.3 Special Topic: Yields of Gaseous Species (NU_GAS)

The yields of fuel and water gases are usually specified with the parameters NU_FUEL and NU_WATER onthe MATL line. However, the yield of any explicitly-defined (via the SPEC line) gas species can be specifiedusing the parameter NU_GAS(j,k), where j refers to the j’th reaction of the material and k refers to thek’th explicitly-defined gas species. NU_GAS can also be used to specify the yields of the mixture fractionand water vapor, assuming each is implicitly-defined and its order in the list of species is obtained from theoutput file CHID.out.

For consistency, the HEAT_OF_COMBUSTION(j,k) can also be specified separately for each reactionand species. These values are used only if the corresponding heat of combustion for the gaseous species isgreater than zero.

In the example below, the pyrolysis of wood is included within a simulation that uses a finite-ratereaction instead of the default mixture fraction model. Notice in this case that all of the gas species (exceptfor the background nitrogen) are explicitly defined, and as a result, FDS needs to be told explicitly whatgaseous species are produced by the solid phase reactions. In this case, 82 % of the mass of wood isconverted to gaseous ’PYROLYZATE’ and 18 % is converted to solid ’CHAR’.

&SPEC ID = 'PYROLYZATE', MW=53.6 /

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&SPEC ID = 'OXYGEN', MASS_FRACTION_0 = 0.23 /&SPEC ID = 'WATER VAPOR' /&SPEC ID = 'CARBON DIOXIDE' /

&MATL ID = 'WOOD'EMISSIVITY = 0.9CONDUCTIVITY = 0.2SPECIFIC_HEAT = 1.3DENSITY = 570.N_REACTIONS = 1A = 1.89E10E = 1.51E5N_S = 1.0NU_RESIDUE = 0.18NU_GAS(1,1:4) = 0.82,0,0,0HEAT_OF_REACTION = 430.HEAT_OF_COMBUSTION(1,1) = 14500.RESIDUE = 'CHAR' /

11.2.4 Special Topic: Finite-Rate Combustion

Usually, FDS uses mixture fraction concepts to describe combustion. However, FDS can also explicitlytrack gas species and reactions that can occur between them. This section describes how to do this.

1. It is strongly recommended that finite-rate reactions be invoked only when FDS is running in DNSmode. Set DNS=.TRUE. on the MISC line. Note: you may use the finite-rate reaction scheme in an LEScalculation, but because the temperature in a large scale calculation is smeared out over a mesh cell,some of the reaction parameters may need to be modified to account for the lower temperatures.

2. The BACKGROUND_SPECIES on the MISC line is normally set to be ’NITROGEN’.

3. The namelist group SPEC is used to specify each additional species. Do not enter a SPEC line for thebackground species.

4. Read Section 11.2 for a description of the boundary conditions for the gas species.

5. The REAC namelist group is used to designate the fuel and the reaction rate parameters. For a finite-ratereaction you can specify multiple REAC lines. Note that FDS will evaluate the reactions in the order theyare listed in the input file.

FUEL Character string indicating which of the listed optional gas species is the fuel.

OXIDIZER Character string indicating which of the listed optional gas species is the oxidizer.

BOF Pre-exponential factor in one-step chemical reaction in units of cm3/mole/s.

E Activation energy for one-step chemical reaction in units of kJ/kmol.

NU Array containing the stoichiometry of the chemical reaction for each SPEC where negative valuesindicate reactants and positive values indicate products. Note that the background species cannotparticipate in the reaction. This means that NU(0) is not a valid input parameter.

N_S Array containing the exponents for the finite rate equation for each SPEC. Note that it is possiblethat a given SPEC can be assigned a value of N_S greater than zero and a value of NU equal to zero.In other words, the rate equation can be dependent on a species that does not participate directly inthe reaction. Note also that the background species cannot participate in the reaction. This meansthat N_S(0) is not a valid input parameter.

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HEAT_OF_COMBUSTION The effective heat of combustion the chemical reaction in units of kJ/kg. (De-fault 40,000 kJ/kg)

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11.3 Radiation Transport: The RADI Namelist Group

For most FDS simulations, thermal radiation transport is computed by default and you need not set anyparameters to make this happen. However, there are situations where it is important to be aware of issuesrelated to the radiative transport solver. The most important issue involves the fraction of energy releasedfrom the fire as thermal radiation, commonly referred to as the radiative fraction. It is a function of boththe flame temperature and chemical composition, neither of which are reliably calculated in a large scalefire calculation because the flame sheet is not well-resolved. In calculations in which the mesh cells areon the order of a centimeter or larger, the temperature near the flame surface cannot be relied upon whencomputing the source term in the radiation transport equation, especially because of the T 4 dependence.As a practical alternative, the parameter RADIATIVE_FRACTION on the RADI line allows you to specifyexplicitly the fraction of the total combustion energy that is released in the form of thermal radiation. Someof that energy may be reabsorbed elsewhere, yielding a net radiative loss from the fire or compartment thatis less than the RADIATIVE_FRACTION, depending mainly on the size of the fire and the soot loading. If itis desired to use the radiation transport equation as is, then RADIATIVE_FRACTION ought to be set to zero,and the source term in the radiative transport equation is then based solely on the gas temperature and thechemical composition. By default, the RADIATIVE_FRACTION is 0.35 for an LES calculation, and zero forDNS.

There are several ways to improve the performance of the Finite Volume Method in solving the radiationtransport equation (RTE), most of which increase the computation time. The solver has two modes of opera-tion – a gray gas model (default) and a wide band model [6]. Modifications to these models can be made viaparameters on the RADI line. If running in gray gas mode (default), increase the number of angles from thedefault 100 with the integer parameter NUMBER_RADIATION_ANGLES. The frequency of calls to the radia-tion solver can be reduced from every 3 time steps with integer called TIME_STEP_INCREMENT. The incre-ment over which the angles are updated can be reduced from 5 with the integer called ANGLE_INCREMENT.Briefly, if TIME_STEP_INCREMENT and ANGLE_INCREMENT are both set to 1, the radiation field is com-pletely updated in a single time step, but the cost of the calculation increases significantly.

A few parameters affecting the absorption of radiation by water droplets are as follows: RADTMP is theassumed radiative source temperature. It is used in the computation of the mean scattering and absorptioncross sections of water droplets. The default is 900 C. NMIEANG is the number of angles in the numericalintegration of the Mie-phase function. Increasing NMIEANG improves the accuracy of the radiative propertiesof water droplets. The cost of the better accuracy is seen in the initialization phase, not during the actualsimulation. The default value for NMIEANG is 15.

If the optional six band model is desired, set WIDE_BAND_MODEL=.TRUE.. It is recommended that thisoption only be used when the fuel is relatively non-sooting because it adds significantly to the cost of the cal-culation. To add three additional fuel bands, set CH4_BANDS=.TRUE.. See FDS Technical Reference Guidefor more details. Note also that when WIDE_BAND_MODEL=.TRUE., the ABSORPTION_COEFFICIENT out-put quantity becomes practically useless, because it then corresponds to one individual band of the spectrum.

It is possible to turn off the radiation transport solver (saving roughly 20 % in CPU time) by addingthe statement RADIATION=.FALSE. to the MISC line. For isothermal calculations, the radiation is turnedoff automatically. If burning is taking place and radiation is turned off, then the total heat release rate isreduced by the RADIATIVE_FRACTION, which is input on the RADI line. This radiated energy completelydisappears from the calculation. More on this feature can be found in Section 11.1.2.

In simulations with no combustion nor radiating species, it is possible to use a constant absorptioncoefficient by specifying KAPPA0 on the RADI line.

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Chapter 12

Particles and Droplets

Lagrangian particles1 are used in FDS to represent water or liquid fuel droplets, flow tracers, and variousother objects that are not defined or confined by the numerical mesh. Sometimes the particles have mass,sometimes they do not. Some evaporate, absorb radiation, etc. PART is the namelist group that is used toprescribe parameters associated with Lagrangian particles.

All Lagrangian particles must be explicitly defined via the PART namelist group. In versions of FDS priorto 5, water droplets and smoke particles were implicitly defined. Shortcuts for defining water droplets andsmoke particles are possible, via parameters like WATER=.TRUE. and MASSLESS=.TRUE.

12.1 Basics

Properties of different types of Lagrangian particles are designated via the PART namelist group. Once aparticular type of particle or droplet has been described using a PART line, then the name of that particleor droplet type is invoked elsewhere in the input file via the parameter PART_ID. There are no reservedPART_IDs – all must be defined. For example, an input file may have several PART lines that include theproperties of different types of Lagrangian particles:

&PART ID='my smoke',... /&PART ID='my water',... /

These Lagrangian particles can be introduced at a solid surface via the SURF line that defines the propertiesof the material, for example

&SURF ...,PART_ID='my smoke' /

or the PART_ID can be invoked from a PROP line to change the properties of the droplets ejected by asprinkler or nozzle, for example

&PROP ID='Acme Spk-123', QUANTITY='SPRINKLER LINK TEMPERATURE', PART_ID='my water', ... /

1Throughout this section, the terms “droplets” and “particles” are used interchangeably. From the point of view of FDS, theyare all Lagrangian particles; that is, point elements that are not bound by the structure of the underlying grid.

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12.2 Particle and Droplet Insertion

There are three ways of introducing droplets or particles into a simulation. The first is to define a sprinkleror nozzle using a PROP line that includes a PART_ID that specifies the droplet parameters. The second wayto introduce particles or droplets is to add a PART_ID to a SURF line, in which case particles or droplets willbe ejected from that surface. Note that this only works if the surface has a normal velocity pointing into theflow domain. The third way to introduce droplets or particles is via an INIT that defines a volume withinthe computational domain in which the particles/droplets are to be introduced initially and/or periodically intime.

12.2.1 Particles Introduced at a Solid Surface

If the particles have mass and are introduced from a solid surface, specify PARTICLE_MASS_FLUX on theSURF line. The number of particles inserted at each solid cell every DT_INSERT seconds is specified byNPPC on the SURF line defining the solid surface. The default value of NPPC is 1. As an example, thefollowing set of input lines:

&PART ID='drops', QUANTITIES(1:3)='DROPLET_DIAMETER','DROPLET_TEMPERATURE','DROPLET_AGE',DIAMETER=750., SAMPLING_FACTOR=1, COLOR='RED', EVAPORATE=.FALSE. /

&SURF ID='HOLE', PART_ID='drops', VEL=-5., PARTICLE_MASS_FLUX=0.1, COLOR='RED' /&OBST XB=-0.2,0.2,-0.2,0.2,4.0,4.4, SURF_IDS='INERT','HOLE','INERT' /

creates an obstruction that ejects non-evaporating, red particles with a mean volumetric diameter of 750 µmout of its sides at a rate of 0.1 kg/m2/s. FDS will adjust the mass flux if the obstruction or vent dimensionsare changed to conform to the numerical grid. Note that the IDs have no meaning other than as identifiers.The particles are colored red in Smokeview, but can also be colored according to their diameter, temperature,or age.

Note that a surface on which particles are specified must have a non-zero normal velocity directed intothe computational domain. This happens automatically if the surface is burning, but must be specified if itis not.

Note also that you can independently control particles that emanate from a solid surface. For example,a device might control the activation of a fan, but you can over-ride the device and control the particlesseparately. To do this, specify either a device or controller via a DEVC_ID or CTRL_ID on the PART line thatdefines the particles. For more information on devices and controls, see Sections 13.4 and 13.5.

12.2.2 Droplets Introduced at a Sprinkler or Nozzle

DROPLETS_PER_SECOND is the number of droplets inserted every second per active sprinkler or nozzle(Default 5000). It is listed on the PROP line that includes other properties of the sprinkler or nozzle. Notethat this parameter only affects sprinklers and nozzles. Changing this parameter does not change the flowrate, but rather the number of droplets used to represent the flow. Also note that the number of dropletsintroduced per “batch” is DROPLETS_PER_SECOND times DT_INSERT.

Note that DROPLETS_PER_SECOND can be a very important parameter. In some simulations, it is a goodidea to increase this number so that the liquid mass is distributed more uniformly over the droplets. Ifthis parameter is too small, it can lead to a non-physical evaporation pattern, sometimes even to the pointof causing a numerical instability. If you encounter a numerical instability shortly after the activation of asprinkler or nozzle, consider increasing DROPLETS_PER_SECOND to produce a smoother evaporation patternthat is more realistic. Keep in mind that for a real sprinkler or nozzle, there are many more droplets createdper second than the number that can be simulated.

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12.2.3 Particles or Droplets Introduced within a Volume

Sometimes it is convenient to introduce droplets or particles at the start of the simulation. For this purpose,add NUMBER_INITIAL_DROPLETS to the INIT line2 to indicate the number of particles/droplets within thecomputational domain at the start of the simulation. Its default value is 0, meaning that initially there areno particles or droplets present. If non-zero, also specify MASS_PER_VOLUME (kg/m3) which specifies theparticle/droplet mass per unit volume (Default 1 kg/m3). Do not confuse this parameter with DENSITY,explained in the next section. For example, water has a DENSITY of 1000 kg/m3, whereas a liter of waterbroken up into droplets and spread over a cubic meter has a MASS_PER_VOLUME of 1 kg/m3. Also, to limitthe particles/droplets to a certain region of the domain, add the real sextuplet XB to designate the coordinatesof a rectangular volume. The format for XB is the same as that used on the OBST line.

&PART ID='drops', DIAMETER=750., SAMPLING_FACTOR=1, COLOR='RED', EVAPORATE=.FALSE. /&INIT PART_ID='drops', XB=..., NUMBER_INITIAL_DROPLETS=1000, MASS_PER_VOLUME=3.5 /&INIT PART_ID='drops', XB=..., NUMBER_INITIAL_DROPLETS=2000, MASS_PER_VOLUME=2.5 /

If you want to introduce droplets or particles within a given volume periodically in time and not just atthe initial time, set DT_INSERT on the INIT line to a positive value indicating the time increment (s)for insertion. The parameter NUMBER_INITIAL_DROPLETS now indicates the number of droplets/particlesinserted every DT_INSERT seconds. If the droplets/particles have mass, use MASS_PER_TIME (kg/s) insteadof MASS_PER_VOLUME to indicate how much mass is to be introduced per second.

12.2.4 Controlling the Number of Particles and Droplets

Regardless of how the particles or droplets are introduced into the computational domain, the following areimportant parameters for controlling their number:

DT_INSERT Time increment in seconds between the introduction of a “batch” of particles or droplets. Thenumber per “batch” depends on how they are introduced. If more particles are desired, lower the inputvalue of this parameter. The default value is 0.01 s. Note that this parameter should be specified on theSURF, PROP, or INIT line, depending on whether the particles/droplets originate at a surface, a sprinkleror nozzle, or a volume. Versions prior to FDS 5.5 had this parameter listed on the PART line.

SAMPLING_FACTOR Sampling factor for the output file CHID.prt5. This parameter can be used to reducethe size of the particle output file used to animate the simulation. The default value is 1 for MASSLESSparticles, meaning that every particle or droplet will be shown in Smokeview. The default is 10 for allother types of particles. MASSLESS particles are discussed in Section 12.4.

AGE Number of seconds the particle or droplet exists, after which time it is removed from the calcula-tion. This is a useful parameter to use when trying to reduce the number of droplets or particles in asimulation.

12.3 Particle and Droplet Properties

Lagrangian particles are used to represent a wide variety of objects that cannot be explicitly resolved on thenumerical mesh. As a result, there are a considerable number of parameters that define them, many of whichmay not be applicable to a particular type of particle or droplet.

2Prior to FDS 5.5, it was possible to use the PART line to specify NUMBER_INITIAL_DROPLETS.

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12.3.1 Thermal Properties

For Lagrangian particles that are not MASSLESS, the following parameters should be included on the PARTline to indicate how they heat up and possibly evaporate. It is assumed by default that non-massless particlesare liquid droplets, but you can specify EVAPORATE=.FALSE. to change this.

DENSITY The density of the liquid or solid droplet/particle. (Default 1000 kg/m3)

SPECIFIC_HEAT Specific heat of liquid or solid droplet/particle.

VAPORIZATION_TEMPERATURE Boiling temperature of liquid droplet.

MELTING_TEMPERATURE Melting (solidification) temperature of liquid droplet.

INITIAL_TEMPERATURE Initial temperature of liquid droplet.

HEAT_OF_VAPORIZATION Latent heat of vaporization of liquid droplet.

H_V_REFERENCE_TEMPERATURE The temperature corresponding to the provided HEAT_OF_VAPORIZATION.

Notice that the default DENSITY is that of water. If the drops are specified as WATER or the drops arespecified asL FUEL and the FUEL species is shown as a liquid in Table 11.1, then only the DENSITY needsto be provided.

12.3.2 Size Distribution

The DIAMETER is the median volumetric diameter of the droplets or particles, with the distribution assumedto be a combination of Rosin-Rammler and log-normal (Default 500 µm). The width of the distribution iscontrolled by the parameter GAMMA_D (default 2.4) The Rosin-Rammler/log-normal distribution is given by

F(d) =

1√2π

∫ d

0

1σd′ e

− [ln(d′/dm)]2

2σ2 dd′ (d ≤ dm)

1− e−0.693( ddm )γ

(dm < d)(12.1)

Note that the parameter σ is given the value σ = 2/(√

2π(ln 2) γ) = 1.15/γ which ensures that the twofunctions are smoothly joined at d = dm. You can also add a value for SIGMA_D to the PART line if youwant to over-ride this feature. The larger the value of γ, the narrower the droplet size is distributed aboutthe median value. Note that you can prevent droplets or particles from exceeding MAXIMUM_DIAMETER,which is infinitely large by default. Also note that droplets less than MINIMUM_DIAMETER are assumed toevaporate in a single time step, eliminating numerical instabilities that can occur when droplets get very,very small. The default MINIMUM_DIAMETER is 20 µm. To prevent FDS from generating a distribution ofdroplets/particles altogether, set MONODISPERSE=.TRUE. on the PART line, in which case every droplet orparticle will be assigned the same DIAMETER.

12.3.3 Drag

For massive particles the default drag law (i.e., the drag coefficient correlation as a function of Reynoldsnumber [based on particle diameter]) is that of a sphere. To invoke the cylinder drag law set DRAG_LAW=‘CYLINDER’on the PART line. If neither of these options is applicable, the user may specify a constant value of the dragcoefficient for a particle class (a specific PART_ID) by setting a USER_DRAG_COEFFICIENT on PART. TheUSER_DRAG_COEFFICIENT trumps the DRAG_LAW.

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12.3.4 Velocity on Solid Surfaces

When a droplet strikes a solid surface, it sticks and is reassigned a new speed and direction. If the sur-face is horizontal, the direction is randomly chosen. If vertical, the direction is downwards. The rate atwhich the droplets move over the horizontal and vertical surfaces is difficult to quantify. The parametersHORIZONTAL_VELOCITY and VERTICAL_VELOCITY on the PART line allow you to control the rate atwhich droplets move horizontally or vertically (downward). The defaults are 0.2 m/s and 0.5 m/s, respec-tively.

There are some applications, like the suppression of racked storage commodities, where it is useful toallow water droplets to move horizontally along the underside of a solid object. It is difficult to model thisphenomenon precisely because it involves surface tension, surface porosity and absorption, and complicatedgeometry. However, a way to capture some of the effect is to set ALLOW_UNDERSIDE_DROPLETS=.TRUE.on the MISC line. It is normally false.

Be aware that when droplets hit obstructions, the vertical direction is assumed to coincide with the z axis,regardless of any change to the gravity vector, GVEC.

A useful sample case to demonstrate various features of droplet motion on solid obstructions is the test casecalled cascade.fds. Figure 12.1 shows a stream of water droplets impinging on the top of a box followed bythe cascading of water droplets over the top edge.

Figure 12.1: Smokeview rendering of the cascade test case.

12.3.5 Color

The parameter QUANTITIES is an array of character strings indicating which scalar quantities should beused to color the particles or droplets when viewed as an animation. The choices are’DROPLET TEMPERATURE’ (C)’DROPLET DIAMETER’ (µm)’DROPLET VELOCITY’ (m/s)’DROPLET MASS’ (kg)’DROPLET AGE’ (s)As a default, if no QUANTITIES are specified and none are selected in Smokeview, then Smokeview will

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display particles with a single color. To select this color specify either RGB or COLOR. By default, waterdroplets are colored blue and fuel droplets yellow. All others are colored black.

12.4 Special Types of Particles and Droplets

There are several useful attributes that you can assign to particles or droplets, usually via a simple logicalparameter. Be aware with each of these parameters that specifying it as .TRUE. may cause other parametersto be functionally useless, or may cause conflicts that FDS may or may not detect. A good rule of thumb isalways to ask yourself what is the basic information that must be conveyed to the program, and stick to that.For example, if the particles are to be MASSLESS, there is no point in declaring thermal properties becausethey are only to be used a flow tracers in Smokeview.

12.4.1 Massless Particles

The simplest use of Lagrangian particles is for visualization, in which case the particles are consideredmassless tracers. In this case, the particles are defined via the line

&PART ID='tracers', MASSLESS=.TRUE., ... /

Note that if the particles are MASSLESS, it is not appropriate to color them according to any particularproperty. Unlike early versions of FDS, particles are no longer colored by gas phase quantities, but rather byproperties of the particle itself. For example, ’DROPLET_TEMPERATURE’ for a non-massless particle refersto the temperature of the particle itself rather than the local gas temperature.

Also note that if MASSLESS=.TRUE., the SAMPLING_FACTOR is set to 1 unless you say otherwise,which would be pointless since MASSLESS particles are for visualization only.

12.4.2 Static Particles or Droplets

STATIC is a logical parameter indicating whether particles move or just serve as obstructions or clutter. Set-ting STATIC=.TRUE. only makes sense in conjunction with a non-zero value of NUMBER_INITIAL_DROPLETSon the INIT line. The default value of STATIC is .FALSE.

12.4.3 Water Droplets

WATER=.TRUE. declares that the liquid droplets evaporate into ’WATER VAPOR’, a separate gas speciesthat is automatically added to the calculation by this command. By default, WATER=.FALSE., even thoughthe default properties of droplets are that of water. Setting WATER=.TRUE. instructs FDS to add ’WATER

VAPOR’ as an explicitly defined species, and it also invokes appropriate constants related to the absorptionof thermal radiation by the water droplets. It also causes the droplets to be colored blue in Smokeview.

If the liquid droplets are to evaporate into some other gaseous species, you must explicitly define thespecies via the SPEC namelist group (see Section 11.2), and then designate the appropriate SPEC_ID on thePART line.

12.4.4 Fuel Droplets

FUEL=.TRUE. indicates that the liquid droplets evaporate into fuel gas and burn. In this case, add theHEAT_OF_COMBUSTION (kJ/kg) of the fuel to the PART line. Fuel droplets are colored yellow by default inSmokeview. This feature only works for a mixture fraction-based combustion calculation, in which case thedroplets evaporate into an equivalent amount of fuel vapor such that the resulting heat release rate (assuming

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complete combustion) is equal to the evaporation rate multiplied by the HEAT_OF_COMBUSTION. If a spraynozzle is used to generate the fuel droplets, its characteristics are specified in the same way as those for asprinkler.

Example Case: spray_burner

Controlled fire experiments are often conducted using a spray burner, where a liquid fuel is sprayed out ofa nozzle and ignited. In this example (spray_burner.fds), heptane from two nozzles is sprayed downwardsinto a steel pan. The flow rate is increased linearly so that the fire grows to 2 MW in 20 s, burns steadily foranother 20 s, and then ramps down linearly in 20 s. The key input parameters are given here:

&DEVC ID='nozzle_1', XYZ=4.0,-.3,0.5, PROP_ID='nozzle', QUANTITY='TIME', SETPOINT=0. /&DEVC ID='nozzle_2', XYZ=4.0,0.3,0.5, PROP_ID='nozzle', QUANTITY='TIME', SETPOINT=0. /

&PART ID='heptane droplets', FUEL=.TRUE., VAPORIZATION_TEMPERATURE=98.,HEAT_OF_VAPORIZATION=316., SPECIFIC_HEAT=2.25, DENSITY=688.,QUANTITIES(1:2)='DIAMETER','DROPLET_TEMPERATURE',DIAMETER=1000., HEAT_OF_COMBUSTION=44500., SAMPLING_FACTOR=1 /

&PROP ID='nozzle', CLASS='NOZZLE', PART_ID='heptane droplets',FLOW_RATE=1.96, FLOW_RAMP='fuel', DROPLET_VELOCITY=10.,SPRAY_ANGLE=0.,30. /

&RAMP ID='fuel', T= 0.0, F=0.0 /&RAMP ID='fuel', T=20.0, F=1.0 /&RAMP ID='fuel', T=40.0, F=1.0 /&RAMP ID='fuel', T=60.0, F=0.0 /

Many of these parameters are self-explanatory. Note that a 2 MW fire is achieved via 2 nozzles flowingheptane at 1.96 L/min each:

2×1.96L

min× 1

60min

s×688

kgm3 ×

11000

m3

L×44500

kJkg

= 2000 kW (12.2)

The parameter HEAT_OF_COMBUSTION over-rides that for the overall reaction scheme. Thus, if otherdroplets or solid objects have different heats of combustion, the effective burning rates are adjusted so thatthe total heat release rate is that which the user expects. However, exercises like this ought to be conductedjust to ensure that this is the case. The HRR curve for this example is given in Fig. 12.2.

Note that FUEL=.TRUE. automatically invokes a mixture fraction calculation in which fuel from theevaporating fuel droplets is burned according to the overall reaction scheme. Because the default mixturefraction combustion model assumes that fuel and oxygen burn when mixed (assuming that the oxygen con-centration is above an empirically determined threshold), there is no need to specify an ignition source. Formost liquid fuels, the small amount of evaporation at ambient temperature is enough to start the combustionprocess. In some sense, there is an implicit pilot flame.

Note also that this feature is subject to mesh dependence. If the mesh cells are too coarse, the evaporatingfuel can be diluted to such a degree that it may not burn. Proper resolution depends on the type of fuel andthe amount of fuel being ejected from the nozzle. Always test your burner at the resolution of your overallsimulation.

12.4.5 Solid Particles that do not Evaporate

Unless you declare MASSLESS=.TRUE. on the PART line, it is assumed that the particle or droplet has massand thermal properties that govern its heating and evaporation. To prevent evaporation, set EVAPORATE=.FALSE.

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0 10 20 30 40 50 600

500

1000

1500

2000

2500

Time (s)

Hea

tR

elea

seR

ate

(kW

)

Heat Release Rate (spray burner)

Specified (HRR)FDS (HRR)

Figure 12.2: Heat Release Rate (HRR) of spray burner test.

The particles will still heat up due to convection, but they will not shrink and no additional gaseous speciesneed to be declared.

Note that the absorption of thermal radiation by water (WATER=.TRUE.) or fuel droplets (FUEL=.TRUE.)is handled in FDS with fairly well-established physical sub-models, the details of which are contained inthe FDS Technical Reference Guide [6]. However, for arbitrary particles or droplets, there is no assumedradiative absorption.

12.5 Special Topic: Suppression by Water (Mixture Fraction Model Only)

Modeling suppression of a fire by a water spray is challenging because the relevant physical mechanismsoccur at length scales smaller than a single mesh cell. In the gas phase, flames are extinguished due tolowered temperatures and dilution of the oxygen supply. See Section 11.1.2 for more information about gasphase suppression.

For the solid phase, water reduces the fuel pyrolysis rate by cooling the fuel surface and also changingthe chemical reactions that liberate fuel gases from the solid. If the solid or liquid fuel has been givenreaction parameters via the MATL line, there is no need to set any additional suppression parameters. It isassumed that water impinging on the fuel surface takes energy away from the pyrolysis process and therebyreduces the burning rate of the fuel. If the surface has been assigned a HRRPUA (Heat Release Rate Per UnitArea), a parameter needs to be specified that governs the suppression of the fire by water because this typeof simulated fire essentially acts like a gas burner whose flow rate is explicitly specified. An empirical wayto account for fire suppression by water is to characterize the reduction of the pyrolysis rate in terms of anexponential function. The local mass loss rate of the fuel is expressed in the form

m′′f (t) = m′′f ,0(t) e−∫

k(t) dt (12.3)

Here m′′f ,0(t) is the user-specified burning rate per unit area when no water is applied and k is a function of

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the local water mass per unit area, m′′w, expressed in units of kg/m2.

k(t) = E_COEFFICIENT m′′w(t) s−1 (12.4)

The parameter E_COEFFICIENT must be obtained experimentally, and it is expressed in units of m2/kg/s.Usually, this type of suppression algorithm is invoked when the fuel is complicated, like a cartoned com-modity.

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Chapter 13

Devices and Control Logic

Sprinklers, smoke detectors, heat flux gauges, and thermocouples may seem to be completely unrelated,but from the point of view of FDS, they are simply devices that operate in specific ways depending on theproperties assigned to them. They can be used to record some quantity of the simulated environment, like athermocouple, or they can represent a mathematical model of a complex sensor, like a smoke detector, andin some cases they can trigger events to happen, like a timer.

Versions of FDS prior to FDS 5 used device-specific namelist groups, like SPRK, HEAT, SMOD, and THCP,but the number and variety of fire-specific sensing and measurement devices continues to expand, and thedata structures in FDS could not easily accommodate all possibilities. In addition, the logic associated withsensor activation and subsequent actions, like a vent opening, had become too complicated and prone tobugs. Starting in FDS 5, all devices, in the broadest sense of the word, are designated via the namelist groupDEVC. In addition, advanced functionality and properties are accommodated via additional namelists groupscalled CTRL (Control) and PROP (Properties).

13.1 Device Location and Orientation: The DEVC Namelist Group (Table15.4)

Regardless of the specific properties, each device needs to be sited either at a point within the computationaldomain, or over a span of the domain, like a beam smoke detector. For example, a sprinkler is sited withinthe domain with a line like:

&DEVC XYZ=3.0,5.6,2.3, PROP_ID='Acme Sprinkler 123', ID='Spk_39' /

The physical coordinates of the device are given by a triplet of real numbers, XYZ. The properties of thedevice are contained on the PROP line designated by PROP_ID, which will be explained below for each ofthe special devices included in FDS. The character string ID is merely a descriptor to identify the device inthe output files, and if any action is tied to its activation.

Not all devices need to be associated with a particular set of properties via the PROP_ID. For example,pointwise output quantities are specified with a single DEVC line, like

&DEVC ID='TC-23', XYZ=3.0,5.6,2.3, QUANTITY='TEMPERATURE' /

which tells FDS to record the temperature at the given point as a function of time. The ID is a label in theoutput file whose name is CHID_devc.csv.

Some devices have a particular orientation which can be specified with various parameters; IOR, ORIENTATION,ROTATION. IOR or the Index of Orientation, is necessary for any device that is placed on the surface of a

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solid. The values ±1 or ±2 or ±3 indicate the direction that the device “points”, where 1 is parallel to thex-axis, 2 is parallel to the y-axis and 3 is parallel to the z-axis.

ORIENTATION is used for devices that are not on a surface and require a directional specification, like asprinkler. ORIENTATION is specified with a triplet of real number values that indicate the components of thedirection vector. The default value of ORIENTATION is (0,0,-1). For example, a default downward-directedsprinkler spray can be redirected in other direction. If you were to specify

&DEVC XYZ=3.0,5.6,2.3, PROP_ID='...', ID='...', ORIENTATION=1,0,0 /

the sprinkler would point in the positive x direction. For other devices, the ORIENTATION would onlychange the way the device is drawn by Smokeview.

13.2 Device Output

Each device has a QUANTITY associated with it. The output file for all DEVC quantities is a comma-delimitedASCII file called CHID_devc.csv (See Section 19.3 for output file format). This file can be imported intomost spread sheet software packages. If the number of DEVC lines exceeds 256, the limit of some spreadsheetapplications, the output file will be split into appropriately sized smaller files. To prevent the file splitting,specify COLUMN_DUMP_LIMIT=.FALSE. on the DUMP line.

By default, the DEVC output is written to a file every DT_DEVC seconds. This time increment is specifiedon the DUMP line. Also, by default, a time-averaged value is written out for each quantity of interest. Toprevent FDS from time-averaging the DEVC output, add TIME_AVERAGED=.FALSE. to the DEVC line.

All devices must have a specified QUANTITY. Some special devices (Section 13.3) have their QUANTITYspecified on a PROP line. A QUANTITY specified on a PROP line associated with a DEVC line will override aQUANTITY specified on the DEVC line.

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13.3 Special Devices and their Properties: The PROPNamelist Group (Table15.18)

Many devices are fairly easy to describe, like a point measurement, with only a few parameters which canbe included on the DEVC line. However, for more complicated devices, it is inconvenient to list all of theproperties on each and every DEVC line. For example, a simulation might include hundreds of sprinklers,but it is tedious to list the properties of the sprinkler each time the sprinkler is sited. For these devices, usea separate namelist group called PROP to store the relevant parameters. Each PROP line is identified by aunique ID, and invoked by a DEVC line by the string PROP_ID. The ID might be the manufacturer’s name,like ’ACME Sprinkler 123’, for example.

The best way to describe the PROP group is to list the various special devices and their properties.

13.3.1 Sprinklers

Here is a very simple example of sprinkler:

&PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', RTI=148., C_FACTOR=0.7,ACTIVATION_TEMPERATURE=74., OFFSET=0.10,PART_ID='water drops', FLOW_RATE=189.3,DROPLET_VELOCITY=10., SPRAY_ANGLE=30.,80. /

&DEVC ID='Spr_60', XYZ=22.88,19.76,7.46, PROP_ID='K-11' /&DEVC ID='Spr_61', XYZ=22.88,21.76,7.46, PROP_ID='K-11' /

A sprinkler, known as ’Spr_60’, is located at a point in space given by XYZ. It is a ’K-11’ type sprinkler,whose properties are given on the PROP line. Note that the various names (IDs) mean nothing to FDS,except as a means of associating one thing with another, so try to use IDs that are as meaningful to you aspossible. The parameter QUANTITY=’SPRINKLER LINK TEMPERATURE’ does have a specific meaningto FDS, directing it to compute the activation of the device using the standard RTI algorithm. The varioussprinkler properties will be discussed below. 1

Properties associated with sprinklers included in the PROP group are:

RTI Response Time Index in units of√

m · s. (Default 100.)

C_FACTOR in units of√

m/s. (Default 0.)

ACTIVATION_TEMPERATURE in units of C. (Default 74 C)

INITIAL_TEMPERATURE of the link in units of C. (Default TMPA)

FLOW_RATE in units of L/min. An alternative is to provide the K_FACTOR in units of L/min/bar12 and the

OPERATING_PRESSURE in units of atm. The flow rate is then given by mw = K√

p. Note that 1 bar isequivalent to 14.5 psi, 1 gpm is equivalent to 3.785 L/min, 1 gpm/psi

12 is equivalent to 14.41 L/min/bar

12 .

OFFSET Radius of a sphere (m) surrounding the sprinkler where the water droplets are initially placed inthe simulation. It is assumed that at and beyond the OFFSET the droplets have completely broken upand are transported independently of each other. (Default 0.05 m)

DROPLET_VELOCITY Initial droplet velocity. (Default 0 m/s)

1Prior to FDS 5, a separate file was used to store properties of a given sprinkler. This file is no longer used.

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ORIFICE_DIAMETER Diameter of the nozzle orifice in m (default 0 m). This input provides an alternativeway to set droplet velocity by giving values for FLOW_RATE and ORIFICE_DIAMETER, in which casethe the droplet velocity is computed by dividing the flow rate by the orifice area. Use this method ifyou do not have any information about droplet velocity. However, quite often the user must fine-tuneDROPLET_VELOCITY in order to reproduce certain spray profile. The ORIFICE_DIAMETER input is notused if either DROPLET_VELOCITY or SPRAY_PATTERN_TABLE is specified.

SPRAY_ANGLE A pair of angles (in degrees) through which the droplets are sprayed. The angles outline aconical spray pattern relative to the south pole of the sphere centered at the sprinkler with radius OFFSET.For example, SPRAY_ANGLE=30.,80. directs the water droplets to leave the sprinkler through a bandbetween 60 and 10 south latitude, assuming the orientation of the sprinkler is (0,0,-1), the default. Thedroplets are uniformly distributed within this belt.

SPRAY_PATTERN_TABLE Name of a set of TABL lines containing the description of the spray pattern.

PART_ID The name of the PART line containing properties of the droplets. See Chapter 12 for additionaldetails.

PRESSURE_RAMP The name of the RAMP lines specifying the dependence of pipe pressure on the numberof active sprinklers and nozzles.

Be aware that sprinklers produce many droplets that need to be tracked in the calculation. To limit thecomputational cost, sprinkler droplets disappear when they hit the lower boundary of the computationaldomain, regardless of whether it is solid or not. To stop FDS from removing sprinkler droplets from the lowerboundary of the computational domain, add the phrase POROUS_FLOOR=.FALSE. to the MISC (Section 6.4)line. Be aware, however, that droplets that land on the floor continue to move horizontally in randomlyselected directions; bouncing off obstructions, and consuming CPU time.

For more information about sprinklers, their activation and spray dynamics, read the FDS TechnicalReference Guide [6].

Special Topic: Specifying Complex Spray Patterns

If a more complex spray pattern is desired than can be achieved by using SPRAY_ANGLE, DROPLET_VELOCITY,and FLOW_RATE, then a SPRAY_PATTERN_TABLE can be specified using the TABL (Section 10) namelistgroup. For a spray pattern, specify the total flow using FLOW_RATE of the PROP line, the name of the spraypattern using SPRAY_PARTTERN_TABLE and then one or more TABL lines of the format:

&TABL ID='table_id', TABLE_DATA=LAT1,LAT2,LON1,LON2,VELO,FRAC /

where each TABL line for a given ’table_id’ provides information about the spherical distribution of thespray pattern for a specified solid angle. LAT1 and LAT2 are the bounds of the solid angle measured indegrees from the south pole (0 is the south pole and 90 is the equator, 180 is the north pole). Note that thisis not the conventional way of specifying a latitude, but rather a convenient system based on the fact thata typical sprinkler sprays water downwards, which is why 0 degrees is assigned to the “south pole,” or the−z direction. The parameters LON1 and LON2 are the bounds of the solid angle (also in degrees), where 0(or 360) is aligned with the −x axis and 90 is aligned with the −y axis. VELO is the velocity (m/s) of thedroplets at their point of insertion. FRAC the fraction of the total flow rate of liquid that should emerge fromthat particular solid angle.

In the test case called bucket_test_2, the spray pattern is defined as two jets, each with a velocity of10 m/s and a flow rate of 60 L/min. The first jet contains 0.2 of the total flow, the second, 0.8 of the total.The jets are centered at points 30 below the “equator,” and are separated by 180.

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&PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', OFFSET=0.10, PART_ID='water_drops',FLOW_RATE=60.,SPRAY_PATTERN_TABLE='TABLE1', SMOKEVIEW_ID='sprinkler_upright',DROPLET_VELOCITY=10. /

&TABL ID='TABLE1',TABLE_DATA=30,31,0,1,5,0.2/&TABL ID='TABLE1',TABLE_DATA=30,31,179,180,5,0.8/

Note that each set of TABL lines must have a unique ID. Also note that the TABL lines can be specified inany order.

Special Topic: Varying Pipe Pressure

In real sprinkler systems, the pipe pressure is affected by the number of actuated sprinklers. Typically, thepressure is higher than the design value when the first sprinkler activates, and decreases towards the designvalue and below that when more and more sprinklers are activated. The pipe pressure has an effect on flowrate, droplet velocity and droplet size distribution.

In FDS, the varying pipe pressure can be specified on a PROP line using PRESSURE_RAMP. On eachRAMP line, the number of open sprinklers or nozzles is associated with certain pipe pressure (atm). Forexample:

&PROP ID='My nozzle'OFFSET=0.1PART_ID='water drops'FLOW_RATE=0.9OPERATING_PRESSURE = 10.0DROPLET_VELOCITY=15.0SPRAY_ANGLE=0.0,80.0PRESSURE_RAMP = 'PR1' /

&RAMP ID = 'PR1' T = 1, F = 16.0 /&RAMP ID = 'PR1' T = 2, F = 10.0 /&RAMP ID = 'PR1' T = 3, F = 8.0 /

These lines would indicate that the pressure is 16.0 atm when the first sprinkler having properties of Mynozzle activates, 10.0 atm when two sprinklers are active, and 8.0 atm when three or more sprinklers areactive. When counting the number of active sprinklers, FDS accounts for all active sprinklers or nozzleswith PART_ID associated with them.

When pressure ramps are used, both FLOW_RATE and DROPLET_VELOCITY are associated with theOPERATING_PRESSURE. Specify either the FLOW_RATE, or the K_FACTOR and OPERATING_PRESSURE.In the latter case, the flow rate is given by mw = K

√p and droplet velocity by v = C

√p. If spray pattern

table is used, the coefficient C is determined separately for each line of the table. The median diameter ofthe particle size distribution is scaled as dm(p) = dm(po)(po/p)1/3, where po is the OPERATING_PRESSUREand dm(po) is specified by parameter DIAMETER on the corresponding PART line.

13.3.2 Nozzles

Nozzles are very much like sprinklers, only they do not activate based on the standard RTI model. Tosimulate a nozzle that activates at a given time, specify a QUANTITY and SETPOINT directly on the DEVCline. The following lines:

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&DEVC XYZ=23.91,21.28,0.50, PROP_ID='nozzle', ORIENTATION=0,0,1, QUANTITY='TIME',SETPOINT=0., ID='noz_1' /

&DEVC XYZ=26.91,21.28,0.50, PROP_ID='nozzle', ORIENTATION=0,0,1, QUANTITY='TIME',SETPOINT=5., ID='noz_2' /

&PROP ID='nozzle', PART_ID='heptane drops', FLOW_RATE=2.132,FLOW_TAU=-50., DROPLET_VELOCITY=5., SPRAY_ANGLE=0.,45. /

designate two nozzles of the same type, one which activates at time zero, the other at 5 s. Note that nozzlesmust have a designated PROP_ID, and the PROP line must have a designated PART_ID to describe the liquiddroplets.

Example Case: flow_rate

This example demonstrates the use of pressure ramps and control logic for opening and closing nozzles. Italso serves as a verification test for the water flow rate. There are four nozzles that open at designated times:0 s, 15 s, 30 s and 45 s. At time 60 s, all the nozzles are closed. The number of open nozzles is measuredusing a device with quantity ’OPEN NOZZLES’. A comparison of the FDS result and the exact, intendedvalues is shown in left part of Figure 13.1.

The pressure ramp has been designed to deliver a total flow rate of 10 l/min at all values of open nozzles.Mathematically this means that

NnK√

p = 10 l/min⇒ p =(

10 l/minNnK

)2

(13.1)

where Nn is the number of open nozzles. The corresponding nozzle and pressure ramp definitions are

&PROP ID='Head',OFFSET=0.10,PART_ID='water drops',K_FACTOR = 2.5OPERATING_PRESSURE = 1PRESSURE_RAMP = 'PR'DROPLET_VELOCITY=2.,SPRAY_ANGLE= 0.,60.,SMOKEVIEW_ID='sprinkler_upright' /

&RAMP ID='PR', T= 1., F=16. /&RAMP ID='PR', T= 2., F=4. /&RAMP ID='PR', T= 3., F=1.778 /&RAMP ID='PR', T= 4., F=1. /

The accumulated water is tracked using a device measuring the accumulated mass per unit area, inte-grated over the total floor area. The total mass of accumulated water should increase from zero to 10 kg in60 s. A comparison of the FDS prediction and this analytical result is shown in the right side of Figure 13.1.The small delay of the FDS result is caused by the time it takes from the droplets to fall down on the floor.

13.3.3 Heat Detectors

QUANTITY=’LINK TEMPERATURE’ defines a heat detector, which uses essentially the same activation al-gorithm as a sprinkler, without the water spray.

&DEVC ID='HD_66', PROP_ID='Acme Heat', XYZ=2.3,4.6,3.4 /&PROP ID='Acme Heat', QUANTITY='LINK TEMPERATURE', RTI=132., ACTIVATION_TEMPERATURE=74. /

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0 10 20 30 40 50 60 700

1

2

3

4

5

Time (s)

Open

Noz

zles

Open Nozzles (flow rate)

Analytical (Water)FDS (Nozzles)

0 10 20 30 40 50 60 700

2

4

6

8

10

12

Time (s)

Mas

s(k

g)

Accumulated Water (flow rate)

Analytical (Water)FDS (Water)

Figure 13.1: Output of the flow_rate test case.

Like a sprinkler, RTI is the Response Time Index in units of√

m · s. ACTIVATION_TEMPERATURE is thelink activation temperature in degrees C (Default 74 C). INITIAL_TEMPERATURE is the initial temperatureof the link in units of C (Default TMPA).

13.3.4 Smoke Detectors

A smoke detector is defined in the input file with an entry similar to:

&DEVC ID='SD_29', PROP_ID='Acme Smoke Detector', XYZ=2.3,4.6,3.4 /&PROP ID='Acme Smoke Detector', QUANTITY='CHAMBER OBSCURATION', LENGTH=1.8,

ACTIVATION_OBSCURATION=3.28 /

for the single parameter Heskestad model. Note that a PROP line is mandatory for a smoke detector, in whichcase the DEVC QUANTITY can be specified on the PROP line. For the four parameter Cleary model, use aPROP line like:

&PROP ID='Acme Smoke Detector I2',QUANTITY='CHAMBER OBSCURATION',ALPHA_E=1.8,BETA_E=-1.1,ALPHA_C=1.0,BETA_C=-0.8,ACTIVATION_OBSCURATION=3.28 /

where the two characteristic filling or “lag” times are of the form:

δte = αeuβe ; δtc = αcuβc (13.2)

The default detector parameters are for the Heskestad model with a characteristic LENGTH of 1.8 m. Forthe Cleary model, the ALPHAs and BETAs must all be listed explicitly. Suggested constants for unidentifiedionization and photoelectric detectors presented in Table 13.1. ACTIVATION_OBSCURATION is the thresh-old value in units of %/m. The threshold can be set according to the setting commonly provided by themanufacturer. The default setting is 3.28 %/m (1 %/ft).

Defining Smoke

By default, FDS assumes that the smoke from a fire is generated in direct proportion to the heat releaserate. A value of SOOT_YIELD=0.01 on the REAC line means that the smoke generation rate is 0.01 of the

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Table 13.1: Suggested Values for Smoke Detector Model. See Ref. [11] for others.

Detector αe βe αc, L βc

Cleary Ionization I1 2.5 -0.7 0.8 -0.9Cleary Ionization I2 1.8 -1.1 1.0 -0.8Cleary Photoelectric P1 1.8 -1.0 1.0 -0.8Cleary Photoelectric P2 1.8 -0.8 0.8 -0.8Heskestad Ionization — — 1.8 —

fuel burning rate. The “smoke” in this case is not explicitly tracked by FDS, but rather is assumed to be afunction of the mixture fraction.

Suppose, however, that you want to define your own “smoke” and that you want to specify its productionrate independently of the HRR (or even in lieu of an actual fire, like a smouldering source). You might alsowant to define a mass extinction coefficient for the smoke and an assumed molecular weight (as it will betracked like a gas species). Finally, you also want to visualize the smoke using the “SMOKE3D” feature inSmokeview. Use the following lines:

&SPEC ID='MY SMOKE', MW=29., MASS_EXTINCTION_COEFFICIENT=8700. /&SURF ID='SMOULDER', TMP_FRONT=1000., MASS_FLUX(1)=0.0001, COLOR='RED' /&VENT XB=0.6,1.0,0.3,0.7,0.0,0.0, SURF_ID='SMOULDER' /

&PROP ID='Acme Smoke', QUANTITY='CHAMBER OBSCURATION', SPEC_ID='MY SMOKE' /&DEVC XYZ=1.00,0.50,0.95, PROP_ID='Acme Smoke', ID='smoke_1' /

&DUMP SMOKE3D_QUANTITY='MY SMOKE', DT_PL3D=30. /

The same smoke detector model is used that was described above. Only now, the mass fraction of yourspecies ’MY SMOKE’ is used in the algorithm, rather than that associated with the mixture fraction. Notethat your species will not participate in the radiation calculation. It will merely serve as a surrogate forsmoke. Note also that if you specify explicitly a smoke surrogate, you should set SOOT_YIELD=0 on theREAC line to prevent FDS from including smoke as a mixture fraction species.

13.3.5 Beam Detection Systems

A beam detector can be defined by specifying the endpoints, (x1,y1,z1) and (x2,y2,z2), of the beamand the total percent obscuration at which the detector activates. The two endpoints must lie in the samemesh. FDS determines which mesh cells lie along the linear path defined by the two endpoints. The beamdetector response is evaluated as

Obscuration =

(1− exp

(−Km

N

∑i=1

ρsoot,i ∆xi

))×100 % (13.3)

where i is a mesh cell along the path of the beam, ρsoot,i is the soot density of the mesh cell, ∆xi is thedistance within the mesh cell that is traversed by the beam, and Km is the mass extinction coefficient. Theline in the input file has the form:

&DEVC XB=x1,x2,y1,y2,z1,z2, QUANTITY='PATH OBSCURATION', ID='beam1', SETPOINT=0.33 /

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Since a single linear path cannot span more than one mesh, having a beam detector that crosses multiplemeshes will require post processing. Break the beam detector path into multiple DEVC lines, one for eachmesh that the beam crosses. The total obscuration is given by

Obscuration =

[1−

N

∏i=1

(1−out puti/100)

]×100 % (13.4)

where out puti is the FDS output for the beam detector of the ith path and the symbol, ∏, indicates a productrather than a sum.

Example Case: beam_detector

A 10 m by 10 m by 4 m compartment is filled with smoke, represented as 0.006 kg/kg of the mixture fractionspecies variable, MIXTURE_FRACTION_22. The default soot yield is 0.01 kg/kg, resulting in a uniform sootdensity of 71.9 mg/m3. Using the default mass extinction coefficient of 8700 m2/kg, the optical depth iscalculated to be 0.626 m−1. The compartment has a series of obstructions located at increasing distancefrom the front in increments of 1 m. The correlation for the output quantity VISIBILITY, Eq. (14.9),produces a visibility distance of 4.8 m. When viewing the smoke levels with Smokeview, you should justbarely see the fifth obstacle which is at a distance of 5 m from the front of the compartment. If this isthe case, Smokeview is properly displaying the obscuration of the smoke. Three beam detectors are alsoplaced in the compartment. These all have a path length of 10 m, but are at different orientations withinthe compartment. Using the optical depth of 0.626 m−1 and the path length of 10 m, the expected totalobscuration is 99.81 %, which is the result computed by FDS for each of the three detectors.

Figure 13.2: Output of the beam_detector test case.

13.3.6 Aspiration Detection Systems

An aspiration detection system groups together a series of soot measurement devices. An aspiration systemconsists of a sampling pipe network that draws air from a series of locations to a central point where anobscuration measurement is made. To define such a system in FDS, you must provide the sampling locations,sampling flow rates, the transport time from each sampling location, and if an alarm output is desired, theoverall obscuration “setpoint.” One or more DEVC inputs are used to specify details of the sampling locations,and one additional input is used to specify the central detector:

2In its default mode, the mixture fraction combustion model requires transport equations for two gas species –MIXTURE_FRACTION_1 and MIXTURE_FRACTION_2. The first is just the fuel gas, the second is a combination ofthe products of combustion.

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&DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='soot', ID='soot1', DEVC_ID='asp1',FLOWRATE=0.1, DELAY=20 /

&DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='soot', ID='soot2', DEVC_ID='asp1',FLOWRATE=0.2, DELAY=10 /

...&DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='soot', ID='sootN', DEVC_ID='asp1',

FLOWRATE=0.3, DELAY=30 /

&DEVC XYZ=..., QUANTITY='ASPIRATION', ID='asp1', BYPASS_FLOWRATE=0.4, SETPOINT=0.02 /

where the DEVC_ID is used at each sampling point to reference the central detector, FLOWRATE is the gasflow rate in kg/s, DELAY is the transport time (in seconds) from the sampling location to the central detector,BYPASS_FLOWRATE is the flow rate in kg/s of any air drawn into the system from outside the computationaldomain (accounts for portions of the sampling network lying outside the domain defined by the MESH inputs),and SETPOINT is the alarm threshold obscuration in units of %/m. The output of the aspiration system iscomputed as

Obscuration =(

1− exp(−Km

∑Ni=1 ρsoot,i(t− td,i) mi

∑Ni=1 mi

))×100 %/m (13.5)

where mi is the mass FLOWRATE of the ith sampling location, ρsoot,i(t−td,i) is the soot density at the ith sam-pling location td,i s prior (DELAY) to the current time t, and Km is the MASS_EXTINCTION_COEFFICIENTassociated with visible light.

Example Case: aspiration_detector

A cubical compartment, 2 m on a side has a three sampling location aspiration system. The three locationshave equal flow rates of 0.3 kg/s, and transport times of 50, 100, and 150 s, respectively. No bypass flow rateis specified for the aspiration detector. Combustion products are forced into the bottom of the compartmentat a rate of 1 kg/s. The SOOT_YIELD=0.001. Mass is removed from the top of the compartment at a rateof 1 kg/s. The aspiration detector shows an increasing obscuration over time. There is a delay of slightlyover 50 s in the initial increase which results from the 50 s transport time for the first sampling location plusa short period of time to transport the combustion products to the sampling location. The detector responsehas three plateaus that result from the delay times of the sampling locations. The sampling points are co-located, so each plateau represents an additional one third of the soot being transported to the detector. Thesoot density at the sampling point is 3.493× 10−4 kg/m3. Using this value the plateaus are computed as63.7 %, 86.8 %, and 95.2 %, as seen in Fig. 13.3.

13.3.7 Electrical Cable Failure

Petra Andersson and Patrick Van Hees of the Swedish National Testing and Research Institute (SP) haveproposed that the thermally-induced electrical failure (THIEF) of a cable can be predicted via a simple one-dimensional heat transfer calculation, under the assumption that the cable can be treated as a homogenouscylinder [12]. Their results for PVC cables were encouraging and suggested that the simplification of theanalysis is reasonable and that it should extend to other types of cables. The assumptions underlying theTHIEF model are as follows:

1. The heat penetration into a cable of circular cross section is largely in the radial direction. This greatlysimplifies the analysis, and it is also conservative because it is assumed that the cable is completelysurrounded by the heat source.

130

0 50 100 150 2000

20

40

60

80

100

Time (s)

Obs

cura

tion

(%/m

)

Obscuration (aspiration detector)

Ideal ValueFDS (asp1)

Figure 13.3: Output of aspiration_detector test case.

2. The cable is homogenous in composition. In reality, a cable is constructed of several different types ofpolymeric materials, cellulosic fillers, and a conducting metal, most often copper.

3. The thermal properties – conductivity, specific heat, and density – of the assumed homogenous cableare independent of temperature. In reality, both the thermal conductivity and specific heat of polymersare temperature-dependent, but this information is very difficult to obtain from manufacturers.

4. It is assumed that no decomposition reactions occur within the cable during its heating, and ignitionand burning are not considered in the model. In fact, thermoplastic cables melt, thermosets form a charlayer, and both off-gas volatiles up to and beyond the point of electrical failure.

5. Electrical failure occurs when the temperature just inside the cable jacket reaches an experimentallydetermined value.

Obviously, there are considerable assumptions inherent in the Andersson and Van Hees THIEF model, buttheir results for various polyvinyl chloride (PVC) cables suggested that it may be sufficient for engineeringanalyses of a wider variety of cables. The U.S. Nuclear Regulatory Commission sponsored a study of cablefailure known as CAROLFIRE [13]. The primary project objective of CAROLFIRE was to characterizethe various modes of electrical failure (e.g. hot shorts, shorts to ground) within bundles of power, con-trol and instrument cables. A secondary objective of the project was to develop a simple model to predictthermally-induced electrical failure when a given interior region of the cable reaches an empirically deter-mined threshold temperature. The measurements used for these purposes are described in Volume II of theCAROLFIRE test report. Volume III describes the modeling.

The THIEF model can only predict the temperature profile within the cable as a function of time, givena time-dependent exposing temperature or heat flux. The model does not predict at what temperature thecable fails electrically. This information is gathered from experiment. The CAROLFIRE experimentalprogram included bench-scale, single cable experiments in which temperature measurements were made onthe surface of, and at various points within, cables subjected to a uniform heat flux. These experimentsprovided the link between internal cable temperature and electrical failure. The model can only predict the

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interior temperature and infer electrical failure when a given temperature is reached. It is presumed that thetemperature of the centermost point in the cable is not necessarily the indicator of electrical failure. Thisanalysis method uses the temperature just inside the cable jacket rather than the centermost temperature, asthat is where electrical shorts in a multi-conductor cable are most likely to occur first.

To use the THIEF model in FDS, add lines similar to the following to the input file:

&PROP ID='Acme Cable', QUANTITY='CABLE TEMPERATURE', CABLE_MASS_PER_LENGTH=0.529,CABLE_FAILURE_TEMPERATURE=400., CABLE_DIAMETER=0.0163,CABLE_JACKET_THICKNESS=0.00152 /

&DEVC XYZ=..., ID='Cable 1', PROP_ID='Acme Cable', ORIENTATION=0,0,1 /&DEVC XYZ=..., ID='Cable 2', PROP_ID='Acme Cable', ORIENTATION=1,0,1 /

.

.

Most of the terms are self-explanatory, and the logic is similar to that of a sprinkler. The THIEF model isinvoked by the specific use of the QUANTITY=’CABLE TEMPERATURE’. The cable mass per unit length isin units of kg/m, failure temperature in C, diameter in m, jacket thickness in m. Note that you can use anyorientation, rather than just the 6 coordinate directions.

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13.4 Basic Control Logic

Devices can be used to control various actions, like creating and removing obstructions, or activating anddeactivating fans and vents. Every device has an associated QUANTITY, whether it is included directly onthe DEVC line or indirectly on the optional PROP line. Using the DEVC parameter SETPOINT, you can triggeran action to occur when the QUANTITY value passes above, or below, the given SETPOINT. The choiceis dictated by the given TRIP_DIRECTION, which is just a positive or negative integer. The followingparameters dictate how a device will control something:

SETPOINT The value of the device at which its state changes. For a detection type of device (e.g. heat orsmoke) this value is taken from the device’s PROP inputs and need not be specified on the DEVC line.

TRIP_DIRECTION A positive integer means the device will change state when its value increases past thesetpoint and a negative integer means the device will change state when its value decreases past thesetpoint. The default value is +1.

LATCH If this logical value is set to .TRUE. the device will only change state once. The default value is.TRUE..

INITIAL_STATE This logical value is the initial state of the device. The default value is .FALSE. Forexample, if an obstruction associated with the device is to disappear, set INITIAL_STATE=.TRUE.

If you desire to control FDS using more complex logic than can be provided by the use of a single deviceand its setpoint, control functions can be specified using the CTRL input. See Section 13.5 for more on CTRLfunctions. The simplest example of a device is just a timer:

&DEVC XYZ=1.2,3.4,5.6, ID='my clock', QUANTITY='TIME', SETPOINT=30. /

Anything associated with the device via the parameter, DEVC_ID=’my clock’, will change its state at 30 s.For example, if the text were added to an OBST line, that obstruction would change from its INITIAL_STATEof .FALSE. to .TRUE. after 30 s. In other words, it would be created at 30 s instead of at the start of thesimulation. This is a simple way to open a door or window.

13.4.1 Creating and Removing Obstructions

In many fire scenarios, the opening or closing of a door or window can lead to dramatic changes in the courseof the fire. Sometimes these actions are taken intentionally, sometimes as a result of the fire. Within theframework of an FDS calculation, these actions are represented by the creation or removal of solid obstacles,or the opening or closing of exterior vents.

Remove or create a solid obstruction by assigning the character string DEVC_ID the name of a DEVC IDon the OBST line that is to be created or removed. This will direct FDS to remove or create the obstructionwhen the device changes state to .FALSE. or .TRUE., respectively. For example, the lines

&OBST XB=..., SURF_ID='...', DEVC_ID='det2' /..

&DEVC XYZ=..., PROP_ID='...', ID='det1' /&DEVC XYZ=..., PROP_ID='...', ID='det2', INITIAL_STATE=.TRUE. /

will cause the given obstruction to be removed when the specified DEVC changes state.Creation or removal at a predetermined time can be performed using a DEVC that has TIME as its mea-

sured quantity. For example, the following instructions will cause the specified HOLEs and OBSTstructionsto appear/disappear at the various designated times. These lines are part of the simple test case calledcreate_remove.fds.

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&HOLE XB=0.25,0.45,0.20,0.30,0.20,0.30, COLOR='RED', DEVC_ID='timer 1' /&HOLE XB=0.25,0.45,0.70,0.80,0.70,0.80, COLOR='GREEN', DEVC_ID='timer 2' /&OBST XB=0.70,0.80,0.20,0.30,0.20,0.30, COLOR='BLUE', DEVC_ID='timer 3' /&OBST XB=0.70,0.80,0.60,0.70,0.60,0.70, COLOR='PINK', DEVC_ID='timer 4' /

&DEVC XYZ=..., ID='timer 1', SETPOINT= 1., QUANTITY='TIME', INITIAL_STATE=.FALSE./&DEVC XYZ=..., ID='timer 2', SETPOINT= 2., QUANTITY='TIME', INITIAL_STATE=.TRUE. /&DEVC XYZ=..., ID='timer 3', SETPOINT= 3., QUANTITY='TIME', INITIAL_STATE=.FALSE./&DEVC XYZ=..., ID='timer 4', SETPOINT= 4., QUANTITY='TIME', INITIAL_STATE=.TRUE. /

The blue obstruction appears at 3 s because its initial state is false, meaning that it does not exist initially.The pink obstruction disappears at 4 s because it does exist initially. The red hole is created at 1 s becauseit does not exist initially (it is filled in with a red obstruction). The green hole is filled in at 2 s becauseit does exist (as a hole) initially. You should always try a simple example first before embarking on acomplicated creation/removal scheme for obstructions and holes.

To learn how to create and remove obstructions multiple times, see Section 13.5.5 for information aboutthe custom control feature.

Starting with FDS version 5.3.0, an obstruction that makes up the boundary of a “pressure zone” (see Sec-tion 9.6) can be created or removed. The reason for this restriction prior to 5.3 was that abrupt changes inpressure could cause numerical instabilities.

13.4.2 Activating and Deactivating Vents

When a device or control function is applied to a VENT, the purpose is to either activate or deactivate anytime ramp associated with the VENT via its SURF_ID. For example, to control a fan with the device ’det2’,do the following:

&SURF ID='FAN', VOLUME_FLUX=5. /&VENT XB=..., SURF_ID='FAN', DEVC_ID='det2' /&DEVC ID='det2', XYZ=..., QUANTITY='TIME', SETPOINT=30., INITIAL_STATE=.FALSE. /

Note that at 30 s, the “state” of the ’FAN’ changes from .FALSE. to .TRUE., or more simply, the ’FAN’turns on. Since there is no explicit time function associated with the ’FAN’, the default 1 s ramp-up willbegin at 30 s instead of at 0 s.

If in this example INITIAL_STATE=.TRUE., then the fan should “deactivate,” or turn off at 30 s. Essen-tially, “activation” of a VENT causes all associated time functions to be delayed until the device SETPOINTis reached. “Deactivation” of a VENT turns off all time functions. Usually this means that the parameters onthe SURF line are all nullified, so it is a good idea to check the functionality with a simple example.

Until further notice, a ’MIRROR’ or ’OPEN’ VENT should not be activated or deactivated. You can, however,place an obstruction in front of an ’OPEN’ VENT and then create it or remove it to model the closing oropening of a door or window.

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13.5 Advanced Control Functions: The CTRL Namelist Group

There are many systems whose functionality cannot be described by a simple device with a single “setpoint.”Consider for example, a typical HVAC system. It is controlled by a thermostat that is given a temperaturesetpoint. The system turns on when the temperature goes below the setpoint by some amount and then turnsoff when the temperature rises above that same setpoint by some amount. This behavior can not be definedby merely specifying a single setpoint. You must also define the range or “deadband” around the setpoint,and whether an increasing or decreasing temperature activates the system. For the HVAC example, crossingthe lower edge of the deadband activates heating; crossing the upper edge activates cooling.

While HVAC is not the primary purpose of FDS, there are numerous situations where a system respondsto the fire in a non-trivial way. The CTRL input is used to define these more complicated behaviors. A controlfunction will take as input the outputs of one or more devices and/or control functions. In this manner,complicated behaviors can be simulated by making functions of other functions. For most of the controlfunction types, the logical value output of the devices and control functions and the time they last changedstate are used as the inputs.

For any object for which a DEVC_ID can be specified (such as OBST or VENT), a CTRL_ID can bespecified instead. A control is identified by the ID parameter. The inputs to the control are identified by

Table 13.2: Control function types for CTRL

Function Type DescriptionANY Changes state if any INPUTs are .TRUE.ALL Changes state if all INPUTs are .TRUE.ONLY Changes state if and only if N INPUTs are .TRUE.AT_LEAST Changes state if at least N INPUTs are .TRUE.TIME_DELAY Changes state DELAY s after INPUT becomes .TRUE.CUSTOM Changes state based on evaluating a RAMP of the function’s inputDEADBAND Behaves like a thermostatKILL Terminates code execution if its sole INPUT is .TRUE.RESTART Dumps restart files if its sole INPUT is .TRUE.

the INPUT_ID parameter. INPUT_ID would be passed one or more ID strings from either devices or othercontrols.

If you want to design a system of controls and devices that involves multiple changes of state, includethe attribute LATCH=.FALSE. on the relevant DEVC or CTRL input lines. By default, devices and controlsmay only change state once, like a sprinkler activating or smoke detector alarming. LATCH=.TRUE. bydefault for both devices and controls.

13.5.1 Control Functions: ANY, ALL, ONLY, and AT_LEAST

Suppose you want an obstruction to be removed (a door is opened, for example) after any of four smokedetectors in a room has activated. Use input lines of the form:

&OBST XB=..., SURF_ID='...', CTRL_ID='SD' /

&DEVC XYZ=1,1,3, PROP_ID='Acme Smoker', ID='SD_1' /

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&DEVC XYZ=1,4,3, PROP_ID='Acme Smoker', ID='SD_2' /&DEVC XYZ=4,1,3, PROP_ID='Acme Smoker', ID='SD_3' /&DEVC XYZ=4,4,3, PROP_ID='Acme Smoker', ID='SD_4' /&CTRL ID='SD', FUNCTION_TYPE='ANY', INPUT_ID='SD_1','SD_2','SD_3','SD_4',

INITIAL_STATE=.TRUE. /

The INITIAL_STATE of the control function SD is .TRUE., meaning that the obstruction exists initially.The “change of state” means that the obstruction is removed when any smoke detector alarms. By default,the INITIAL_STATE of the control function SD is .FALSE., meaning that the obstruction does not existinitially.

Suppose that now you want the obstruction to be created (a door is closed, for example) after all foursmoke detectors in a room have activated. Use a control line of the form:

&CTRL ID='SD', FUNCTION_TYPE='ALL', INPUT_ID='SD_1','SD_2','SD_3','SD_4' /

The control functions AT_LEAST and ONLY are generalizations of ANY and ALL. For example,

&CTRL ID='SD', FUNCTION_TYPE='AT_LEAST', N=3, INPUT_ID='SD_1','SD_2','SD_3','SD_4' /

changes the state from .FALSE. to .TRUE. when at least 3 detectors activate. Note that in this example, andthe example below, the parameter N is used to specify the number of activated or “TRUE” inputs requiredfor the conditions of the Control Function to be satisfied. The control function,

&CTRL ID='SD', FUNCTION_TYPE='ONLY', N=3, INPUT_ID='SD_1','SD_2','SD_3','SD_4' /

changes the state from .FALSE. to .TRUE. when 3, and only 3, detectors activate.

13.5.2 Control Function: TIME_DELAY

There is often a time delay between when a device activates and when some other action occurs, like in adry pipe sprinkler system.

&DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler_link', QUANTITY='LINK TEMPERATURE',ID='Spk_29_link', CTRL_ID='dry pipe' /

&DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler', QUANTITY='CONTROL', ID='Spk_29',CTRL_ID='dry pipe' /

&CTRL ID='dry pipe', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='Spk_29_link', DELAY=30. /

This relationship between a sprinkler and its pipes means that the sprinkler spray is controlled (in this casedelayed) by the ’dry pipe’, which adds 30 s to the activation time of Spk_29, measured by Spk_29_link,before water can flow out of the head.

13.5.3 Control Function: DEADBAND

This control function behaves like an HVAC thermostat. It can operate in one of two modes analagous toheating or cooling. The function is provided with an INPUT_ID which is the DEVC whose value is used bythe function, a DEADBAND, and the mode of operation by ON_BOUND. If ON_BOUND=’LOWER’, the functionchanges state from its INITIAL_VALUE when the value of the INPUT_ID drops below the lower value inDEADBAND and reverts when it increases past the upper value, i.e. like a heating system. The reverse willoccur if ON_BOUND=’UPPER’, i.e. like a cooling system.

For an HVAC example, the following lines of input would set up a simple thermostat:

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&SURF ID='FAN', TMP_FRONT=40., VOLUME_FLUX=-1. /&VENT XB=-0.3,0.3,-0.3,0.3,0.0,0.0, SURF_ID='FAN', CTRL_ID='thermostat' /&DEVC ID='TC', XYZ=2.4,5.7,3.6, QUANTITY='TEMPERATURE' /&CTRL ID='thermostat', FUNCTION_TYPE='DEADBAND', INPUT_ID='TC',

ON_BOUND='LOWER', SETPOINT=23.,27.,LATCH=.FALSE./

Here, we want to control the VENT that simulates the FAN, which blows hot air into the room. A DEVC

called TC is positioned in the room to measure the TEMPERATURE. The thermostat uses a SETPOINT toturn on the FAN when the temperature falls below 23 C (ON_BOUND=’LOWER’) and it turns off when thetemperature rises above 27 C.

Note that a deadband controller needs to have LATCH set to .FALSE.

13.5.4 Control Function: RESTART and KILL

There are times when you might only want to run a simulation until some goal is reached. Previously thiscould generally only be done by constantly monitoring the simulation’s output and manually stopping thecalculation when you determine that the goal has been reached. The KILL control function can do thisautomatically.

Additionally there are analyses where you might want to create some baseline condition and then runmultiple permutations of that baseline. For example, you might want to run a series of simulations wheredifferent mitigation strategies are tested once a detector alarms. Using the RESTART control function, youcan cause a restart file to be created once a desired condition is met. The simulation can continue and therestart files can be copied to have the job identifying string, CHID, of the various permutations (providing ofcourse that the usual restrictions on the use of restart files are followed). For example, the lines

&DEVC ID='temp', QUANTITY='TEMPERATURE', SETPOINT=1000., XYZ=4.5,6.7,3.6 /&DEVC ID='velo', QUANTITY='VELOCITY', SETPOINT=10., XYZ=4.5,6.7,3.6 /

&CTRL ID='kill', FUNCTION_TYPE='KILL', INPUT_ID='temp' /&CTRL ID='restart', FUNCTION_TYPE='RESTART', INPUT_ID='velo' /

will “kill” the job and output restart files when the temperature at the given point rises above 1000 C; orjust force restart files to be output when the velocity at a given point exceeds 10 m/s.

13.5.5 Control Function: CUSTOM

For most of the control function types, the logical (true/false) output of the devices and control functionsand the time they last changed state are taken as inputs. A CUSTOM function uses the numerical output of aDEVC along with a RAMP to determine the output of the function. When the RAMP output for the DEVC valueis negative, the CTRL will have the value of its INITIAL_STATE. When the RAMP output for the DEVC valueis positive, the CTRL will have the opposite value of its INITIAL_STATE. In the case below, the CUSTOM

control function uses the numerical output of a timer device as its input. The function returns true (thedefault value for INITIAL_STATE is .FALSE.) when the F parameter in the ramp specified with RAMP_ID

is a positive value and false when the RAMP F value is negative. In this case, the control would start false andwould switch to true when the timer reaches 60 s. It would then stay in a true state until the timer reaches120 s and would then change back to false.

Note that when using control functions the IDs assigned to both the CTRL and the DEVC inputs must beunique across both sets of inputs, i.e. you cannot use the same ID for both a control function and a device.

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In the HVAC example above, we could set the system to function on a fixed cycle by using a CUSTOM controlfunction based on time:

&SURF ID='FAN', TMP_FRONT=40., VOLUME_FLUX=-1. /&VENT XB=-0.3,0.3,-0.3,0.3,0.0,0.0, SURF_ID='FAN', CTRL_ID='cycling timer' /&DEVC ID='TIMER', XYZ=2.4,5.7,3.6, QUANTITY='TIME' /&CTRL ID='cycling timer', FUNCTION_TYPE='CUSTOM, INPUT_ID='TIMER', RAMP_ID='cycle' /&RAMP ID='cycle', T= 59, F=-1 /&RAMP ID='cycle', T= 61, F= 1 /&RAMP ID='cycle', T=119, F= 1 /&RAMP ID='cycle', T=121, F=-1 /

In the above example the fan will be off initially, turn on at 60 s and then turn off at 120 s.You can make an obstruction appear and disappear multiple times by using lines like

&OBST XB=..., SURF_ID='whatever', CTRL_ID='cycling timer' /&DEVC ID='TIMER', XYZ=..., QUANTITY='TIME' /&CTRL ID='cycling timer', FUNCTION_TYPE='CUSTOM', INPUT_ID='TIMER', RAMP_ID='cycle' /&RAMP ID='cycle', T= 0, F=-1 /&RAMP ID='cycle', T= 59, F=-1 /&RAMP ID='cycle', T= 61, F= 1 /&RAMP ID='cycle', T=119, F= 1 /&RAMP ID='cycle', T=121, F=-1 /

The above will have the obstacle initially removed, then added at 60 s, and removed again at 120 s.Experiment with these combinations using a simple case before trying a case to make sure that FDS

indeed is doing what is intended.

13.5.6 Combining Control Functions: A Pre-Action Sprinkler System

For a pre-action sprinkler system, the normally dry sprinkler pipes are flooded when a detection eventoccurs. For this example, the detection event is when two of four smoke detectors alarm. It takes 30 sto flood the piping network. The nozzle is a DEVC named ’NOZZLE 1’ controlled by the CTRL named’nozzle trigger’. The nozzle activates when both detection and the time delay have occurred. Notethat the DEVC is specified with QUANTITY=’CONTROL’.

&DEVC XYZ=1,1,3, PROP_ID='Acme Smoker', ID='SD_1' /&DEVC XYZ=1,4,3, PROP_ID='Acme Smoker', ID='SD_2' /&DEVC XYZ=4,1,3, PROP_ID='Acme Smoker', ID='SD_3' /&DEVC XYZ=4,4,3, PROP_ID='Acme Smoker', ID='SD_4' /&DEVC XYZ=2,2,3, PROP_ID='Acme Nozzle', QUANTITY='CONTROL',

ID='NOZZLE 1', CTRL_ID='nozzle trigger' /

&CTRL ID='nozzle trigger', FUNCTION_TYPE='ALL', INPUT_ID='smokey','delay' /&CTRL ID='delay', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='smokey', DELAY=30. /&CTRL ID='smokey', FUNCTION_TYPE='AT_LEAST', N=2, INPUT_ID='SD_1','SD_2','SD_3','SD_4' /

13.5.7 Combining Control Functions: A Dry Pipe Sprinkler System

For a dry-pipe sprinkler system, the normally dry sprinkler pipes are pressurized with gas. When a linkactivates in a sprinkler head, the pressure drop allows water to flow into the pipe network. For this exampleit takes 30 s to flood the piping network once a sprinkler link has activated. The sequence of events requiredfor operation is first ANY of the links must activate which starts the 30 s TIME_DELAY. Once the 30 s delayhas occurred, each nozzle with an active link, the ALL control functions, will then flow water.

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&DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler Link', ID='LINK 1' /&DEVC XYZ=2,3,3, PROP_ID='Acme Sprinkler Link', ID='LINK 2' /

&PROP ID='Acme Sprinkler Link', QUANTITY='LINK TEMPERATURE',ACTIVATION_TEMPERATURE=74., RTI=30./

&DEVC XYZ=2,2,3, PROP_ID='Acme Nozzle', QUANTITY='CONTROL',ID='NOZZLE 1', CTRL_ID='nozzle 1 trigger' /

&DEVC XYZ=2,3,3, PROP_ID='Acme Nozzle', QUANTITY='CONTROL',ID='NOZZLE 2', CTRL_ID='nozzle 2 trigger' /

&CTRL ID='check links', FUNCTION_TYPE='ANY', INPUT_ID='LINK 1','LINK 2'/&CTRL ID='delay', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='check links', DELAY=30. /&CTRL ID='nozzle 1 trigger', FUNCTION_TYPE='ALL', INPUT_ID='delay','LINK 1'/&CTRL ID='nozzle 2 trigger', FUNCTION_TYPE='ALL', INPUT_ID='delay','LINK 2'/

13.5.8 Example Case: activate_vents

The simple test case called activate_vents demonstrates the several of the control functions. Figure 13.4shows seven multiply-colored vents that activate at different times, depending on the particular timing orcontrol function.

Figure 13.4: Output of the activate_vents test case at 5, 10 and 15 s.

13.6 Controlling a RAMP

For any user defined RAMP, the normal independent variable, for example time for RAMP_V, can be replacedby the output of a DEVC. This is done by specifying the input DEVC_ID one on of the RAMP input lines. Inthe following example a blower is turned on when the temperature exceeds 100 C and is turned off whenthe temperature exceeds 200 C. This is similar functionality to the CUSTOM control function, but it allowsfor variable response rather than just on or off.

&SURF ID='BLOWER',VEL=-2,RAMP_V='BLOWER RAMP'/&DEVC XYZ=2,3,3, QUANTITY='TEMPERATURE',ID='TEMP DEVC'/&RAMP ID='BLOWER RAMP', T=20,F=0,DEVC_ID='TEMP DEVC'/&RAMP ID='BLOWER RAMP', T=100,F=0/&RAMP ID='BLOWER RAMP', T=101,F=1/&RAMP ID='BLOWER RAMP', T=200,F=1/&RAMP ID='BLOWER RAMP', T=201,F=0/

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13.7 Visualizing FDS Devices Using Smokeview Objects

Smokeview generates visual representations of FDS devices using instructions found in a data file namedobjects.svo. These instructions correspond to OpenGL library calls, the same type of calls Smokeviewuses to visualize FDS cases. New objects may be designed and drawn without modifying Smokeview andmore importantly may be created by any user not just the FDS/Smokeview developers. This section givesan overview of Smokeview objects detailing what objects are available and how to modify them. Furtherdocumentation giving underlying technical details may be found in the Smokeview User’s Guide [2].

Smokeview objects may be static or dynamic. A static object is defined entirely in terms of data andinstructions found in the objects.svo file. For example, the sensor object is static, it is drawn as asmall green sphere with a fixed diameter. Its appearance remains the same regardless of how an FDS inputfile is set up. A dynamic object is also defined using instructions found in objects.svo but in additionuses data specified on the &PROP namelist statement and/or data contained in a particle file. As a result, theappearance of dynamic objects depends on the particular FDS case that is run. For example, the tsphereobject is dynamic. The diameter and an image used to cover the sphere (known as a texture map) is specifiedin an FDS input file.

13.7.1 Static Smokeview Objects

Smokeview objects consist of one or more frames or views. Smokeview then displays FDS devices in anormal/inactive state or in an active state. A sprinkler, for example, is drawn differently depending onwhether it has activated or not. When FDS determines that a device has activated it places a message inthe .smv file indicating the object number, the activation time and the state (0 for inactive or 1 for active).Smokeview then draws the corresponding frame. Tables 13.3 and 13.4 give a list of various static objects.Each entry shows an image of the object in its normal/inactive state and in its active state if it has one.The intersection of black tubes indicate the origin, the part of the device displayed at the (x,y,z) coordinatespecified on the &DEVC input line.

The SMOKEVIEW_ID keyword found on the &PROP namelist statement is used to associate an FDSdevice with a Smokeview object. The following FDS input file lines were used to display the target devicein Table 13.3.

&PROP ID='target' SMOKEVIEW_ID='target' /&DEVC XYZ=0.5,0.8,0.6, QUANTITY='TEMPERATURE' PROP_ID='target' /

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Table 13.3: Single Frame Static Objects

SMOKEVIEW_ID Image

sensor

target

Table 13.4: Dual Frame Static Objects

SMOKEVIEW_IDImage

inactive active

heat_detector

nozzle

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Table 13.4: Dual Frame Static Objects (continued)

SMOKEVIEW_IDImage

inactive active

smoke_detector

sprinkler_upright

sprinkler_pendent

13.7.2 Dynamic Smokeview Objects - Customized Using &PROP Parameters

The appearance of several Smokeview objects may be modified using data specified on the &PROP nameliststatement in an FDS input file. Objects may also be customized using data stored in a particle file. This isdiscussed in the next section.

The SMOKEVIEW_PARAMETERS keyword on the &PROP namelist statement is used to customize theappearance of Smokeview objects. For example, the &DEVC and &PROP statements:

&PROP ID='sphere' SMOKEVIEW_PARAMETERS(1:4)='R=0','G=255','B=0','D=0.5' SMOKEVIEW_ID='sphere' /

&DEVC XYZ=0.5,0.8,1.5, QUANTITY='TEMPERATURE' PROP_ID='sphere' /

create an FDS device drawn as a sphere colored green with diameter 0.5. Each parameter specified usingthe SMOKEVIEW_PARAMETERS keyword is a text string enclosed in single quotes. The text string is of the

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form ’keyword=value’where possible keywords are found in the objects.svo file (labels beginningwith ‘:’). For example, R, G, B and D may be used as keywords to customize the following sphere object:

OBJECTDEF // object for particle file spheresphere:R=0 :G=0 :B=0 :D=0.1$R $G $B setrgb$D drawsphere

Another, Smokeview object, the tsphere, uses a texture map or picture to alter the appearance of theobject. The texture map is specified using SMOKEVIEW_PARAMETERS keyword by placing the characterst% before the texture file name (e.g. t%texturefile.jpg).

Table 13.5 gives a list of dynamic objects and the keyword/parameter pairs used to specify them. Eachentry shows an image of the object and the parameters used to customize its appearance.

Table 13.5: Dynamic Objects - Customized using SMOKEVIEW_PARAMETERS on a &PROP line

SMOKEVIEW_ID SMOKEVIEW_PARAMETERS Image

ball

SMOKEVIEW_PARAMETERS(1:6)=’R=128’,’G=192’,’B=255’,’DX=0.5’,’DY=.75’,’DZ=1.0’

R, G, B - red, green, blue color componentsranging from 0 to 255DX, DY, DZ - amount ball is stretched along x, y,z axis respectively

cone

SMOKEVIEW_PARAMETERS(1:5)=’R=128’,’G=255’,’B=192’,’D=0.4’,’H=0.6’

R, G, B - red, green, blue color componentsranging from 0 to 255D, H - diameter and length of cone respectively

fan

SMOKEVIEW_PARAMETERS(1:11)=’HUB_R=0’,’HUB_G=0’,’HUB_B=0’,’HUB_D=0.1’,’HUB_L=0.12’,’BLADE_R=128’,’BLADE_G=64’,’BLADE_B=32’,’BLADE_ANGLE=60.0’,’BLADE_D=0.5’,’BLADE_H=0.09’

HUB_R, HUB_G, HUB_B - red, green, bluecolor components of fan hub ranging from 0 to255HUB_D, HUB_L - diameter and length of fanhubBLADE_R, BLADE_G, BLADE_B - red, green,blue color components of fan blades ranging from0 to 255BLADE_ANGLE, BLADE_D, BLADE_H -angle, diameter and height of a fan blade

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Table 13.5: Dynamic Objects (continued)

SMOKEVIEW_ID SMOKEVIEW_PARAMETERS Image

tsphere

SMOKEVIEW_PARAMETERS(1:9)=’R=255’,’G=255’,’B=255’,’AX0=0.0’,’ELEV0=90.0’,’ROT0=0.0’,’ROTATION_RATE=10.0’,’D=1.0’,’tfile="t%sphere_cover_04.png"’

R, G, B - red, green, blue color componentsranging from 0 to 255AX0, ELEV0, ROT0 - initial azimuth, elevationand rotation angle respectivelyROTATION_RATE - rotation rate about z axis indegrees per secondD - diameter of spheretfile - name of texture map file

vent

SMOKEVIEW_PARAMETERS(1:6)=’R=192’,’G=192’,’B=128’,’W=0.5’,’H=1.0’, ’ROT=90.0’

R, G, B - red, green, blue color componentsranging from 0 to 255W, H - width and height of vent respectivelyROT - angle that vent is rotated

inactive vent

active vent

13.7.3 Dynamic Smokeview Objects - Customized Using &PROP Parameters and ParticleFile Data

Particle file data may be used to customize the appearance of Smokeview objects. Any objects that havecolor labels named R, G, B (including those objects in Table 13.5) may be colored using particle file data.In addition, objects that use variable names that match shortened particle file quantity names3 may be cus-tomized. This data may be used to alter the geometry or structure of the object using particle file data. Forexample, U-VEL, V-VEL, W-VEL and temp are shortened quantity names that correspond to the full namesU-VELOCITY, V-VELOCITY, W-VELOCITY and TEMPERATURE. These full names are documented inTable 14.1 in this guide.

3Short forms of particle file quantity names appear in the Smokeview colorbar label and in the Smokeview File/Bounds dialogbox.

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The first three lines of the velegg object definition are:

OBJECTDEF // color with FDS quantity, stretch with velocityvelegg:R=0 :G=0 :B=0 :D :U-VEL=1.0 :V-VEL=1.0 :W-VEL=1.0

The variables U-VEL, V-VEL, and W-VEL in the above line are also particle file quantities (shortenedversion) that may be selected in an FDS input file. If they are selected, then the velegg object may be usedto display particle file information. This object colors the sphere using the currently selected Smokeviewparticle variable and stretches a sphere in the x, y and z directions using the U-VEL, V-VEL, and W-VELvelocity particle data respectively.

Table 13.6 documents those objects that can be customized using particle file data. These objects may becustomized as before using data specified with the SMOKEVIEW_PARAMETERS keyword or using particlefile data.

Table 13.6: Dynamic Objects - Customized using SMOKEVIEW_PARAMETERS on a &PROP line andParticle File Data

SMOKEVIEW_ID SMOKEVIEW_PARAMETERS Image

box

SMOKEVIEW_PARAMETERS(1:6)=’R=192’,’G=255’,’B=128’,’DX=0.25’,’DY=.5’,’DZ=0.125’

R, G, B - red, green, blue color componentsranging from 0 to 255DX, DY, DZ - amount box is stretched along x, y,z axis respectively

tube

SMOKEVIEW_PARAMETERS(1:5)=’R=255’,’G=0’,’B=0’,’D=0.2’,’L=0.6’

R, G, B - red, green, blue color componentsranging from 0 to 255D, L - diameter and length of tube respectively

velegg

SMOKEVIEW_PARAMETERS(1:9)=:R=0 :G=0 :B=0:U-VEL=1.0 :V-VEL=1.0 :W-VEL=1.0:VELMIN :VELMAX :D

R, G, B - red, green, blue color componentsranging from 0 to 255U-VEL, V-VEL, W-VEL - u, v, w components ofvelocityVELMIN, VELMAX - minimum and maximumvelocityD - diameter of egg at maximum velocity

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Table 13.6: Dynamic Objects (continued)

SMOKEVIEW_ID SMOKEVIEW_PARAMETERS Image

veltube

SMOKEVIEW_PARAMETERS(1:9)=:R=0 :G=0 :B=0:U-VEL=1.0 :V-VEL=1.0 :W-VEL=1.0:VELMIN :VELMAX :D

R, G, B - red, green, blue color componentsranging from 0 to 255U-VEL, V-VEL, W-VEL - u, v, w components ofvelocityVELMIN, VELMAX - minimum and maximumvelocityD - diameter of tube at VELMAX

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Chapter 14

Output Data

Before a calculation is started, carefully consider what information should be saved. All output quantitiesmust be specified at the start of the calculation. In most cases, there is no way to retrieve information afterthe calculation ends if it was not specified from the start. There are several different ways of visualizing theresults of a calculation. Most familiar to experimentalists is to save a given quantity at a single point in spaceso that this quantity can be plotted as a function of time, like a thermocouple temperature measurement. Thenamelist group DEVC, described previously, is used to specify point measurements.

To visualize the flow patterns better, save planar slices of data, either in the gas or solid phases, by usingthe SLCF (SLiCe File) or BNDF (BouNDary File) namelist group. Both of these output formats permit youto animate these quantities in time.

For static pictures of the flow field, use the PLot3D files that are automatically generated 5 times a run.Plot3D format is used by many CFD programs as a simple way to store specified quantities over the entiremesh at one instant in time.

Finally, tracer particles can be injected into the flow field from vents or obstacles, and then viewed inSmokeview. Use the PART namelist group to control the injection rate, sampling rate and other parametersassociated with particles.

Note: unlike in FDS version 1, particles are no longer used to introduce heat into the flow, thus particles nolonger are ejected automatically from burning surfaces.

14.1 Output Control Parameters: The DUMP Namelist Group

The namelist group DUMP contains parameters (Table 15.5) that control the rate at which output files arewritten, and various other global parameters associated with output files. This namelist group is new startingin FDS 5, although its parameters have been specified via other namelist groups in past versions.

NFRAMES Number of output dumps per calculation. The default is 1000. Device data, slice data, parti-cle data, isosurface data, 3D smoke data, boundary data, solid phase profile data, and control functiondata are saved every (T_END-T_BEGIN)/NFRAMES seconds unless otherwise specified using DT_DEVC,DT_SLCF, DT_PART, DT_ISOF, DT_BNDF, DT_PROF, or DT_CTRLNote that DT_SLCF controls Smoke3Doutput. DT_HRR controls the output of heat release rate and associated quantities.

MASS_FILE If .TRUE., produce an output file listing the total masses of all gas species as a function oftime. It is .FALSE. by default because the calculation of all gas species in all mesh cells is time-consuming. The parameter DT_MASS controls the frequency of output.

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STATE_FILE If .TRUE., produce an output file listing the species mass fractions associated with the mix-ture fraction, assuming complete combustion. It is .FALSE. by default, and only makes sense forcalculations involving the mixture fraction model.

MAXIMUM_DROPLETS Maximum number of Lagrangian particles that can be included on any mesh at anygiven time. (Default 500000)

SMOKE3D If .FALSE., do not produce an animation of the smoke and fire. It is .TRUE. by default.

FLUSH_FILE_BUFFERS FDS purges the output file buffers periodically and forces the data to be writtenout into the respective output files. It does this to make it easier to view the case in Smokeview while itis running. It has been noticed on Windows machines that occasionally a runtime error occurs becauseof file access problems related to the buffer flushing. If this happens, set this parameter to .FALSE.,but be aware that it may not be possible to look at output in Smokeview until after the calculation isfinished. You may also set DT_FLUSH to control the frequency of the file flushing. Its default value isthe duration of the simulation divided by NFRAMES.

STATUS_FILES If .TRUE., produces an output file CHID.notready which is deleted, if the simulation iscompleted successfully. This file can be used as an error indicator. It is .FALSE. by default.

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14.2 Output File Types

FDS has various types of output files that store computed data. Some of the files are in binary format andintended to be read and rendered by Smokeview. Some of the files are just comma-delimited text files. Itis important to remember that you must explicitly declare in the input file most of the FDS output data. Aconsiderable amount of the input file is usually devoted to this.

14.2.1 Device Output: The DEVC Namelist Group

For many commonly used measurement devices there is no need to associate a specific PROP line to theDEVC entry. In such cases, use the character string QUANTITY to indicate that a particular gas or solid phasequantity at the point should be recorded in the output file with the suffix _devc.csv. The quantities are listedin Table 14.1. Many of the gas phase quantities are self-explanatory. For example, if you just want to recordthe time history of the temperature at a given point, add

&DEVC XYZ=6.7,2.9,2.1, QUANTITY='TEMPERATURE', ID='T-1' /

and a column will be added to the output file CHID_devc.csv under the label ’T-1’. In this case, the ID hasno other role than as a column label in the output file. Note that versions of FDS prior to version 5 used an8 cell linear interpolation for a given gas phase point measurement. In other words, if you specified a pointvia the triplet of real numbers, XYZ, FDS would calculate the value of the quantity by linearly interpolatingthe values defined at the centers of the 8 nearest cells. Starting in FDS 5, this is no longer done. Instead,FDS reports the value of the QUANTITY in the cell where the point XYZ is located.

When prescribing a solid phase quantity, be sure to position the probe at a solid surface. It is not alwaysobvious where the solid surface is since the mesh does not always align with the input obstruction locations.To help locate the appropriate surface, the parameter IOR must be included when designating a solid phasequantity, except when using the STATISTICS feature described in Section 14.3.10 in which case the outputquantity is not associated with just a single point on the surface. If the orientation of the solid surface is inthe positive x direction IOR=1, negative x direction IOR=-1, positive y IOR=2, negative IOR=-2, positive zIOR=3, and negative z IOR=-3. There are still instances where FDS cannot determine which solid surfaceis being designated, in which case an error message appears in the diagnostic output file. Re-position theprobe and try again. For example, the line

&DEVC XYZ=0.7,0.9,2.1, QUANTITY='WALL TEMPERATURE', IOR=-2, ID='...' /

designates the surface temperature of a wall facing the negative y direction.In addition to point measurements, the DEVC group can be used to report integrated quantities (See

Table 14.1). For example, you may want to know the mass flow out of a door or window. To report this, addthe line

&DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0, QUANTITY='MASS FLOW', ID='whatever' /

Note that in this case, a plane is specified rather than a point. The sextuplet XB is used for this purpose.Notice when a flow is desired, two of the six coordinates need to be the same. Another QUANTITY, HRR, canbe used to compute the total heat release rate within a subset of the domain. In this case, the sextuplet XBought to define a volume rather than a plane. Specification of the plane or volume over which the integrationis to take place can only be done using XB – avoid planes or volumes that cross multiple mesh boundaries.FDS has to decide which mesh to use in the integration, and it chooses the finest mesh overlapping thecentroid of the designated plane or volume.

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14.2.2 Quantities within Solids: The PROF Namelist Group

FDS uses a fine, non-uniform, one-dimensional mesh at each boundary cell to compute heat transfer within asolid. The parameters (Table 15.17) to specify a given PROFile are similar to those used to specify a surfacequantity in the DEVC group. XYZ designates the triplet of coordinates, QUANTITY is the physical quantity tomonitor, IOR the orientation, and ID an identifying character string. Here is an example of how you woulduse this feature to get a time history of temperature profiles within a given solid obstruction:

&PROF XYZ=..., QUANTITY='TEMPERATURE', ID='TU1SA_FDS', IOR=3 /

Other possible quantities are the total density of the wall (QUANTITY = ’DENSITY’) or densities of solidmaterial components (QUANTITY = ’MATL_ID’), where MATL_ID is the name of the material.

Each PROF line creates a separate file. This may be more than is needed. Sometimes, all you want toknow is the temperature at a certain depth. To get an inner wall temperature, you can also just use a deviceas follows:

&DEVC XYZ=..., QUANTITY='INSIDE WALL TEMPERATURE', DEPTH=0.005, ID='Temp_1', IOR=3 /

The parameter DEPTH (m) indicates the distance inside the solid surface. Note that this QUANTITY is allowedonly as a DEVC, not a BNDF, output. Also note that if the wall thickness is decreasing over time due to thesolid phase reactions, the distance is measured from the current surface, and the measurement point is’moving’ towards the back side of the solid. Eventually, the measurement point may get out of the solid,in which case it starts to show ambient temperature. If you just want to know the temperature of the backsurface of the “wall,” then use

&DEVC XYZ=..., QUANTITY='BACK WALL TEMPERATURE', ID='Temp_b', IOR=3 /

Note that this quantity is only meaningful if the front or exposed surface of the “wall” has the attributeBACKING=’EXPOSED’ on the SURF line that defines it. The coordinates, XYZ, and orientation, IOR, referto the front surface. To check that the heat conduction calculation is being done properly, you can add theadditional line

&DEVC XYZ=..., QUANTITY='WALL TEMPERATURE', ID='Temp_f', IOR=-3 /

where now XYZ and IOR refer to the coordinates and orientation of the back side of the wall. These two walltemperatures ought to be the same. Remember that the “wall” in this case can only be at most one mesh cellthick, and its THICKNESS need not be the same as the mesh cell width. Rather, the THICKNESS ought to bethe actual thickness of the “wall” through which FDS performs a 1-D heat conduction calculation.

14.2.3 Animated Planar Slices: The SLCF Namelist Group

The SLCF (“slice file”) namelist group parameters (Table 15.22) allows you to record various gas phasequantities at more than a single point. A “slice” refers to a subset of the whole domain. It can be a line,plane, or volume, depending on the values of XB. The sextuplet XB indicates the boundaries of the “slice”plane. XB is prescribed as in the OBST or VENT groups, with the possibility that 0, 2, or 4 out of the 6values be the same to indicate a volume, plane or line, respectively. A handy trick is to specify, for example,PBY=5.3 instead of XB if it is desired that the entire plane y = 5.3 slicing through the domain be saved. PBXand PBZ control planes perpendicular to the x and z axes, respectively.

Animated vectors can be created in Smokeview if a given SLCF line has the attribute VECTOR=.TRUE. Iftwo SLCF entries are in the same plane, then only one of the lines needs to have VECTOR=.TRUE.Otherwise,a redundant set of velocity component slices will be created.

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Normally, FDS averages slice file data at cell corners. For example, gas temperatures are computed atcell centers, but they are linearly interpolated to cell corners and output to a file that is read by Smokeview.To prevent this from happening, set CELL_CENTERED=.TRUE. This forces FDS to output the actual cell-centered data with no averaging. Note that this feature is mainly useful for diagnostics because it enablesyou to visualize the values that FDS actually computes. Note also that this feature should only be used forscalar quantities that are computed at cell centers, like temperatures, mass fractions, etc.

Slice file information is recorded in files (See Section 19.7) labeled CHID_n.sf, where n is the index ofthe slice file. A short fortran program fds2ascii.f produces a text file from a line, plane or volume of data.See Section 14.4 for more details.

14.2.4 Animated Boundary Quantities: The BNDF Namelist Group

The BNDF (“boundary file”) namelist group parameters allows you to record surface quantities at all solidobstructions. As with the SLCF group, each quantity is prescribed with a separate BNDF line, and the outputfiles are of the form CHID_n.bf. No physical coordinates need be specified, however, just QUANTITY. SeeTable 14.1. For certain output quantities, additional parameters need to be specified via the PROP namelistgroup. In such cases, add the character string, PROP_ID, to the BNDF line to tell FDS where to find thenecessary extra information.

Note that BNDF files (Section 19.9) can become very large, so be careful in prescribing the time interval.One way to reduce the size of the output file is to turn off the drawing of boundary information on desiredobstructions. On any given OBST line, if the string BNDF_OBST=.FALSE. is included, the obstruction is notcolored. To turn off all boundary drawing, set BNDF_DEFAULT=.FALSE. on the MISC line. Then individualobstructions can be turned back on with BNDF_OBST=.TRUE. on the appropriate OBST line. Individualfaces of a given obstruction can be controlled via BNDF_FACE(IOR), where IOR is the index of orientation(+1 for the positive x direction, -1 for negative, and so on).

Normally, FDS averages boundary file data at cell corners. For example, surface temperatures arecomputed at the center of each surface cell, but they are linearly interpolated to cell corners and output to afile that is read by Smokeview. To prevent this from happening, set CELL_CENTERED=.TRUE. on the BNDFline. This forces FDS to output the actual cell-centered data with no averaging. Note that this feature ismainly useful for diagnostics because it enables you to visualize the values that FDS actually computes.

14.2.5 Animated Isosurfaces: The ISOF Namelist Group

The ISOF (“ISOsurface File”) namelist group is used to specify the output of gas phase scalar quantities, asthree dimensional animated contours. For example, a 300 C temperature isosurface shows where the gastemperature is 300 C. Three different values of the temperature can be saved via the line:

&ISOF QUANTITY='TEMPERATURE', VALUE(1)=50., VALUE(2)=200., VALUE(3)=500. /

where the values are in degrees C. Note that the isosurface output files CHID_n.iso can become very large,so experiment with different sampling rates (DT_ISOF on the DUMP line).

Any gas phase quantity can animated via iso-surfaces, but use caution. To render an iso-surface,the desired quantity must be computed in every mesh cell at every output time step. For quantities likeTEMPERATURE, this is not a problem, as FDS computes it and saves it anyway. However, soot density

or oxygen demand substantial amounts of time to compute at each mesh cell.

14.2.6 Plot3D Static Data Dumps

By default, flow field data in Plot3D format is output 5 times a run. Five quantities are written out to a fileat one instant in time. The default specification is:

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&DUMP ..., PLOT3D_QUANTITY(1:5)='TEMPERATURE','U-VELOCITY','V-VELOCITY','W-VELOCITY','HRRPUV' /

It’s best to leave the velocity components as is, because Smokeview uses them to draw velocity vectors. Thefirst and fifth quantities can be changed with the parameters PLOT3D_QUANTITY(1) and PLOT3D_QUANTITY(5)on the DUMP line. If any of the specified quantities require the additional specification of a particular species,use PLOT3D_SPEC_ID(n) to provide the SPEC_ID for PLOT3D_QUANTITY(n).

Note that there can only be one DUMP line.

Data stored in Plot3D [14] files (See Section 19.8) use a format developed by NASA and used bymany CFD programs for representing simulation results. Plot3D data is visualized in three ways: as 2Dcontours, vector plots and iso-surfaces. Vector plots may be viewed if one or more of the u, v and w velocitycomponents are stored in the Plot3D file. The vector length and direction show the direction and relativespeed of the fluid flow. The vector colors show a scalar fluid quantity such as temperature. Plot3D data arestored in files with extension .q . There is an optional file that can be output with coordinate informationif another visualization package is being used to render the files. If you write WRITE_XYZ=.TRUE. on theDUMP line, a file with suffix .xyz is written out. Smokeview does not require this file because the coordinateinformation can be obtained elsewhere.

14.2.7 SMOKE3D: Realistic Smoke and Fire

When you do a fire simulation using the default mixture fraction combustion model, FDS automaticallycreates two output files that are rendered by Smokeview as realistic looking smoke and fire. By default,the output quantities are the ’MASS FRACTION’ of ’soot’ and ’HRRPUV’ (Heat Release Rate Per UnitVolume) are used in the visualization. You have the option of rendering any other species mass fractioninstead of ’soot’, so long as the MASS_EXTINCTION_COEFFICIENT (either from the REAC line, or over-ridden by the value on the SPEC line) is appropriate in describing the attenuation of visible light by thespecified gas species. The alternative gas species is given by SMOKE3D_QUANTITY on the DUMP line. Ifthe specified quantity requires the additional specification of a particular species, use SMOKE3D_SPEC_IDto provide the SPEC_ID. See the Smokeview User’s Guide for more details on how these quantities arerendered.

Here is an example of how to control the smoke species. Normally, you do not need to do this as the“smoke” is an assumed part of the default mixture fraction model.

&SPEC ID='MY SMOKE', MW=29., MASS_EXTINCTION_COEFFICIENT=8700. /&SURF ID='NO FIRE', TMP_FRONT=1000., MASS_FLUX(1)=0.0001, COLOR='RED' /&VENT XB=0.6,1.0,0.3,0.7,0.0,0.0, SURF_ID='NO FIRE' /&DUMP SMOKE3D_QUANTITY='MASS FRACTION', SMOKE3D_SPEC_ID='MY SMOKE' /

The production rate of ’MY SMOKE’ is 0.0001 kg/m2/s, applied to an area of 0.16 m2. TheMASS_EXTINCTION_COEFFICIENT is passed to Smokeview to be used for visualization.

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14.3 Special Output Quantities

This section lists a variety of output quantities that are useful for studying thermally-driven flows, com-bustion, pyrolysis, and so forth. Note that some of the output quantities can be produced in a variety ofways.

14.3.1 Heat Release Rate

Quantities associated with the overall energy budget are reported in the comma delimited file CHID_hrr.csv.This file is automatically generated; the only input parameter associated with it is DT_HRR on the DUMP line.The file consists of six columns. The first column contains the time in seconds. The second through fifthcolumns contain integrated energy gains and losses, all in units of kW. The second column contains the totalheat release rate, the third contains the radiative heat loss to all the boundaries (solid and open), the fourthcontains the convective and radiative heat loss to the boundaries (i.e. the energy flowing out of or into thedomain), and the fifth contains the energy conducted into the solid surfaces. The sixth column contains thetotal burning rate of fuel, in units of kg/s. Note that the reported value of the burning rate is not adjustedto account for the possibility that each individual material might have a different heat of combustion. Forthis reason, it is not always the case that the reported total burning rate multiplied by the gas phase heat ofcombustion is equal to the reported heat release rate.

Let Ω denote the unblocked computational domain, i.e. the volume within the bounding rectangleoccupied by gas. Let ∂Ω by the boundary of Ω. The boundary can be divided into two parts ∂Ω = ∂Ω1 +∂Ω2.The first part ∂Ω1 consists of all the solid walls. The second part ∂Ω2 consists of openings from outside thedomain through which gases may flow. This could be an open window to the exterior, or a forced vent.

The total heat release rate is given by

Q =∫

Ω

q′′′ dV (14.1)

The radiative loss to the boundaries can be computed with either a volume or boundary integral

Qr =∫

Ω

∇ ·qr dV =∫

∂Ω

qr ·dS =∫

∂Ω

q′′r dA (14.2)

It represents the energy radiating away from the fire and hot gases into the solid boundaries or out of thecomputational domain. The convective/radiative loss to open boundaries is

Qconv =∫

∂Ω

cp ρ (T −T∞) u ·dS+∫

∂Ω2

q′′r dA (14.3)

where the integral is positive if the flow and radiative flux are going out of the domain. The conductive lossto solid surfaces is given by

Qcond =∫

∂Ω1

q′′r + q′′c dA (14.4)

where the integral is positive if heat is being lost into a wall colder than the gas.For scenarios in which the fire is the primary source of energy, after the gas temperatures within the

computational domain reach a nearly steady state

Q≈ Qconv + Qcond (14.5)

This is merely a check of the global energy balance, that is, the energy generated within the space heats upthe gases and solid surfaces, and then a balance between heat input and output is achieved.

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Note that, in axially symmetric simulations, the coordinates XB(3) and XB(4) refer to the width of thecylindrical sector at 1 m distance from the cylinder axis, and the volume Ω is the volume of the cylindricalsector

Ω =1

2R1

(XB(2)2−XB(1)2

)(XB(4)−XB(3))(XB(6)−XB(5)) (14.6)

where R1 = 1 m.

14.3.2 Visibility and Obscuration

If you are performing a fire calculation using the mixture fraction approach, the smoke is tracked alongwith all other major products of combustion. The most useful quantity for assessing visibility in a space isthe light extinction coefficient, K [15]. The intensity of monochromatic light passing a distance L throughsmoke is attenuated according to

I/I0 = e−KL (14.7)

The light extinction coefficient, K, is a product of the density of smoke particulate, ρYs, and a mass specificextinction coefficient that is fuel dependent

K = Km ρYs (14.8)

Devices that output a % obscuration such as a DEVCwith a QUANTITY of ASPIRATION, CHAMBER OBSCURATION

(smoke detector), or PATH OBSCURATION (beam detector) are discussed respectively in Section 13.3.6,Section 13.3.4, and Section 13.3.5

Estimates of visibility through smoke can be made by using the equation

S = C/K (14.9)

where C is a non-dimensional constant characteristic of the type of object being viewed through the smoke,i.e. C = 8 for a light-emitting sign and C = 3 for a light-reflecting sign [15]. Since K varies from pointto point in the domain, the visibility S does as well. Keep in mind that FDS can only track smoke whoseproduction rate and composition are specified. Predicting either is beyond the capability of the presentversion of the model.

Three parameter control smoke production and visibility; each parameter is input on the REAC line. Thefirst parameter is SOOT_YIELD, which is the fraction of fuel mass that is converted to soot if the mixturefraction model is being used. The second parameter is called the MASS_EXTINCTION_COEFFICIENT, andit is the Km in Eq. (14.8). The default value is 8700 m2/kg, a value suggested for flaming combustion ofwood and plastics1. The third parameter is called the VISIBILITY_FACTOR, the constant C in Eq. (14.9).It is 3 by default.

The gas phase output quantity ’EXTINCTION COEFFICIENT’ is K. A similar quantity is the ’OPTICALDENSITY’, K/2.3, the result of using log10 in the definition

D≡−1L

log10

(II0

)= K log10 e (14.10)

The visibility S is output via the keyword VISIBILITY. Note that, by default, the visibility is associatedwith the smoke that is implicitly defined by the mixture fraction model. However, this quantity can alsobe associated with an explicitly defined species via the inclusion of a SPEC_ID. In other words, you canspecify the output quantity ’VISIBILITY’ along with a SPEC_ID. This does not require that you do amixture fraction calculation; only that you have specified the given species via a separate SPEC line. Youcan specify a unique MASS_EXTINCTION_COEFFICIENT on the SPEC line as well.

1For most flaming fuels, a suggested value for Km is 8700 m2/kg ± 1100 m2/kg at a wavelength of 633 nm [16]

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14.3.3 Layer Height and the Average Upper and Lower Layer Temperatures

Fire protection engineers often need to estimate the location of the interface between the hot, smoke-ladenupper layer and the cooler lower layer in a burning compartment. Relatively simple fire models, often re-ferred to as two-zone models, compute this quantity directly, along with the average temperature of the upperand lower layers. In a computational fluid dynamics (CFD) model like FDS, there are not two distinct zones,but rather a continuous profile of temperature. Nevertheless, there are methods that have been developedto estimate layer height and average temperatures from a continuous vertical profile of temperature. Onesuch method [17] is as follows: Consider a continuous function T (z) defining temperature T as a functionof height above the floor z, where z = 0 is the floor and z = H is the ceiling. Define Tu as the upper layertemperature, Tl as the lower layer temperature, and zint as the interface height. Compute the quantities:

(H− zint) Tu + zint Tl =∫ H

0T (z) dz = I1

(H− zint)1Tu

+ zint1Tl

=∫ H

0

1T (z)

dz = I2

Solve for zint :

zint =Tl(I1 I2−H2)

I1 + I2 T 2l −2Tl H

(14.11)

Let Tl be the temperature in the lowest mesh cell and, using Simpson’s Rule, perform the numerical integra-tion of I1 and I2. Tu is defined as the average upper layer temperature via

(H− zint) Tu =∫ H

zint

T (z) dz (14.12)

Further discussion of similar procedures can be found in Ref. [18].The quantities LAYER HEIGHT, UPPER TEMPERATURE and LOWER TEMPERATURE can be designated

via “device” (DEVC) lines in the input file2. For example, the entry

&DEVC XB=2.0,2.0,3.0,3.0,0.0,3.0, QUANTITY='LAYER HEIGHT', ID='whatever' /

produces a time history of the smoke layer height at x = 2 and y = 3 between z = 0 and z = 3. If multiplemeshes are being used, the vertical path cannot cross mesh boundaries.

14.3.4 The True Gas Temperature vs. the Measured Gas Temperature

The output quantity THERMOCOUPLE is the temperature of the thermocouple itself, usually close to the gastemperature, but not always. It is determined by solving the following equation for TTC iteratively [19]

εTC(σT 4TC−U/4)+h(TTC−Tg) = 0 (14.13)

where εTC is the emissivity of the thermocouple, U is the integrated radiative intensity, Tg is the truegas temperature, and h is the heat transfer coefficient to a small sphere, h = kaNu/Pr/dTC. The beadBEAD_DIAMETER and BEAD_EMISSIVITY are given on the associated PROP line. See the discussion onheat transfer to a water droplet in the Technical Reference Guide for details of the convective heat transferto a small sphere.

2Note that in FDS 5 and beyond, these quantities are no longer available as slice files.

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14.3.5 Heat Fluxes and Thermal Radiation

There are various ways of recording the heat flux at a solid boundary. If you want to record the net heatflux to the surface, q′′c + q′′r , use the QUANTITY called ’NET HEAT FLUX’. The individual components,the net convective and radiative fluxes, are ’CONVECTIVE HEAT FLUX’ and ’RADIATIVE HEAT FLUX’,respectively. If you want to compare predicted heat flux with a measurement, you often need to use ’GAUGEHEAT FLUX’. The difference between ’NET HEAT FLUX’ and ’GAUGE HEAT FLUX’ is that the former isthe rate at which energy is absorbed by the solid surface; the latter is the amount of energy that would beabsorbed if the surface were cold (or some specified temperature):

q′′r /ε+ q′′c +h(Tw−T∞)+σ(T 4w −T 4

∞)

If the heat flux gauge used in an experiment has a temperature other than ambient, set GAUGE_TEMPERATUREon the PROP line associated with the device. When comparing against a radiometer measurement, useRADIOMETER:

q′′r /ε+σ(T 4w −T 4

∞)

For diagnostic purposes it is sometimes convenient to output the ’INCIDENT HEAT FLUX’:

q′′r /ε+σT 4w + q′′c

All of the above heat flux output quantities are defined at a solid surface. However, ’RADIATIVEHEAT FLUX GAS’ acts like a radiometer that is not attached to a solid surface. This quantity integrates theincoming radiative flux over 2π solid angles around the vector defined by ORIENTATION, a triplet of realnumbers that defines the orientation of the device. For example,ORIENTATION=-1.0,0.0,0.0

means that the device points in the negative x direction. Unlike IOR, the ORIENTATION can be in anydirection, not just those associated with the three coordinate directions.

Note that the sign of the output of heat flux is different than the sign of the input of a heat flux. Apositive output quantity for heat flux means heat is being transferred into the surface.

14.3.6 Droplet Output Quantities

Droplet Quantities on Solid Surfaces

It is possible to record various properties of droplets and particles. Some of the output quantities are as-sociated with solid boundaries. For example, ’MPUA’ is the Mass Per Unit Area of the droplets namedPART_ID. Likewise, ’AMPUA’ is the Accumulated Mass Per Unit Area. Both of these are given in unitsof kg/m2. Think of these outputs as measures of the instantaneous mass density per unit area, and the ac-cumulated total, respectively. The accumulated total is analogous to a “bucket test,” where the droplets arecollected in buckets and the total mass determined at the end of a given time period. The cooling of a solidsurface by droplets of a given type is given by ’CPUA’, the Cooling Per Unit Area in units of kW/m2.

Be aware of the fact that the default behavior for droplets hitting the “floor,” that is, the plane z = ZMIN,is to disappear (POROUS_FLOOR=.TRUE. on the MISC line). In this case, ’MPUA’will be zero, but ’AMPUA’will not. FDS stores the droplet mass just before removing the droplet from the simulation for the purposeof saving CPU time.

In the test case bucket_test, a single sprinkler is mounted 10 cm below a 5 m ceiling. Water flows for30 s at a constant rate of 180 L/min (ramped up and down in 1 s). The simulation continues for another 10 sto allow water drops time to reach the floor. The total mass of water discharged is

180L

min×1

kgL× 1

60min

s×30 s = 90 kg (14.14)

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In the simulation, the boundary quantity water_drops_AMPUA (Accumulated Mass Per Unit Area) recordsthe total water mass per unit area (kg/m2), analogous to actual buckets the size of a grid cell. Summing thevalues of water_drops_AMPUA over the entire floor yields 88.3 kg. Where is the missing water? Somedroplets evaporate, and some droplets fly beyond the computational domain. Note that there are no actual“buckets” in the simulation.

The accumulated water mass at the floor is extracted from the boundary (BNDF) file via the commandline program fds2ascii. Here is a transcript of the session used to convert the binary FDS output file intoASCII format:

>> fds2asciiEnter Job ID string (CHID):

bucket_testWhat type of file to parse?PL3D file? Enter 1SLCF file? Enter 2BNDF file? Enter 3

3Enter Sampling Factor for Data?(1 for all data, 2 for every other point, etc.)

1Limit the domain size? (y or n)

yEnter min/max x, y and z

-5 5 -5 5 0 11 MESH 1, water_drops_AMPUAEnter starting and ending time for averaging (s)

35 36Enter orientation: (plus or minus 1, 2 or 3)

3Enter number of variables

1Enter boundary file index for variable 1

1Enter output file name:

bucket_test_fds2ascii.csvWriting to file... bucket_test_fds2ascii.csv

Note that there really is no need to time-average the results. The quantity is inherently accumulating. Also,the “orientation” refers to direction of the surface. In this case, we’re interested in the positive z direction,or 3.

Droplet Mass and Fluxes in Gas Phase

Away from solid surfaces, ’MPUV’ is the Mass Per Unit Volume of the droplets as they fly through the air, inunits of kg/m3. ’DROPLET FLUX X’, ’DROPLET FLUX Y’, and ’DROPLET FLUX Z’ produce only sliceand Plot3D files of the mass flux of droplets in the x, y, and z directions, respectively, in units of kg/m2/s.

Local Spray Properties

Detailed experimental measurements of sprays are often performed using Phase Doppler Particle Analysis(PDPA) which can be used to obtain information on the droplet size distribution, speed and concentration.Special device type has been defined to simulate the PDPA measurement. The actual quantity to measure,and the details of the measurement are defined using an associated PROPerty. Note that in FDS, the PDPAdevice cannot produce complete droplet size distributions, but only various mean properties.

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The PDPA device output at time t is computed as

f (t) =1

min(t, te)− ts

∫ min(t,te)

ts

(∑i nidm

i x∑i nidn

i

)1/(m−n)

dt (14.15)

where x is the quantity to be measured, or just one in case of mean diameters. The summation goes overall the particles within a sphere with radius PDPA_RADIUS and centered at the location given by the deviceXYZ. The properties of the PDPA device are defined using the following keywords on the PROP line:

PART_ID Name of the particle group to limit the computation to. Do not specify to account for all particles.

PDPA_START ts, starting time of time integration in seconds. PDPA output is always a running average overtime. As the spray simulation may contain some initial transient phase, it may be useful to specify thestarting time of data collection.

PDPA_END te, ending time of time integration in seconds.

PDPA_M m, exponent of diameter.

PDPA_N n, exponent of diameter. In case m = n, the exponent 1/(m−n) is removed from the formula.

PDPA_RADIUS Radius (m) of the sphere, centered at the device location, inside which the particles aremonitored.

QUANTITY Particle property that is used for variable x. Possible variables are ’VELOCITY’,’U-VELOCITY’,’V-VELOCITY’,’W-VELOCITY’,’TEMPERATURE’ and ’NUMBER CONCENTRATION’. Leave unspeci-fied to compute mean diameters.

The following example is used to measure the Sauter mean diameter D32 of the particle type ’water

drops’, starting from time 5.0 s.

&PROP ID='pdpa_d32'PART_ID='water drops'PDPA_M=3PDPA_N=2PDPA_RADIUS=0.01PDPA_START=5.0 /

&DEVC XYZ=0.0,0.0,1.0, QUANTITY='PDPA', PROP_ID='pdpa_d32' /

14.3.7 Interfacing with Structural Models

FDS solves a one-dimensional heat conduction equation for each boundary cell marking the interface be-tween gas and solid, assuming that material properties for the material layer(s) are provided. The results canbe transferred (via either DEVC or BNDF output) to other models that predict the mechanical response of thewalls or structure. For many applications, the 1-D solution of the heat conduction equation is adequate, butin situations where it is not, another approach can be followed. FDS includes a calculation of the AdiabaticSurface Temperature (AST), a quantity that is representative of the heat flux to a solid surface. Following theidea proposed by Ulf Wickstrom [20], the following equation can be solved via a simple iterative techniqueto determine an effective gas temperature, TAST:

q′′r + q′′c = εσ(T 4

AST−T 4w)+h(TAST−Tw) (14.16)

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The sum q′′r + q′′c is the net heat flux onto the solid surface, whose temperature is Tw. The heat fluxes andsurface temperature are computed in FDS, and they are inter-dependent. The computed wall temperatureaffects the net heat flux and vice versa. However, because FDS only computes the solution to the 1-Dheat conduction equation in the solid, it may be prone to error if lateral heat conduction within the solid issignificant. Thus, in some scenarios neither the FDS-predicted heat fluxes or the surface temperature can beused as an accurate indicator of the thermal insult from the hot, smokey gases onto solid objects.

Of course, both the heat fluxes, q′′r and q′′c , and the surface temperature, Tw can be passed from FDS tothe other model, and suitable corrections can be made based on a presumably more accurate prediction of thesolid temperature. Alternatively, the single quantity, TAST, can be transferred, as this is the temperature thatthe solid surface effectively “sees.” It represents the gas phase thermal environment, however complicated,but it does not carry along the uncertainty associated with the simple solid phase heat conduction modelwithin FDS. Obviously, the objective in passing information to a more detailed model is to get a betterprediction of the solid temperature (and ultimately its mechanical response) than FDS can provide.

14.3.8 Useful Solid Phase Outputs

In addition to the PROFile output, there are various additional quantities that are useful for monitoring re-acting surfaces. For example, ’WALL THICKNESS’ gives the overall thickness of the solid surface element.’SURFACE DENSITY’ gives the overall mass per unit area for the solid surface element, computed as anintegral of material density over wall thickness. Both quantities are available both as DEVC and BNDF.

To record the change in a material component’s density with time, use the output quantity ’SOLID

DENSITY’ in the following way:

&DEVC ID='...', XYZ=..., IOR=3, QUANTITY='SOLID DENSITY', MATL_ID='wood', DEPTH=0.001 /

This produces a time history of the density of the material referred to as ’wood’ on a MATL line. The densityis recorded 1 mm beneath the surface which is oriented in the positive z direction. Note that if ’wood’ ispart of a mixture, the density represents the mass of ’wood’ per unit volume of the mixture. Note also that’SOLID DENSITY’ is only available as a DEVC (device) quantity.

14.3.9 Fractional Effective Dose (FED)

The Fractional Effective Dose index (FED), developed by Purser [21], is a commonly used measure ofhuman incapasitation due to the combustion gases. The present version of FDS uses only the concentrationsof the gases CO, CO2, and O2 to calculate the FED value as

FEDtot = FEDCO×HVCO2 +FEDO2 (14.17)

The fraction of an incapacitating dose of CO is calculated as

FEDCO = 4.607×10−7 (CCO)1.036 t (14.18)

where t is time in seconds and CCO is the CO concentration (ppm). The fraction of an incapacitating dose oflow O2 hypoxia is calculated as

FEDO2 =t

60exp [8.13−0.54(20.9−CO2)](14.19)

where CO2 is the O2 concentration (volume per cent). The hyperventilation factor induced by carbon dioxideis calculated as

HVCO2 =exp(0.1930CCO2 +2.0004)

7.1(14.20)

159

where CCO2 is the CO2 concentration (percent).Note that the spatial integration features (Section 14.3.10) cannot be used with FED output because FED

makes use of the TIME INTEGRAL (Section 14.3.11). For the same reason, FED output is only available asa point measurement.

14.3.10 Spatially-Integrated Outputs

A useful feature of a device (DEVC) is to specify an output quantity along with a desired statistic. Forexample,

&DEVC XB=2.3,4.5,2.8,6.7,3.6,7.8, QUANTITY='TEMPERATURE', ID='maxT', STATISTICS='MAX' /

causes FDS to write out the maximum gas phase temperature over the volume bounded by XB. Note thatit does not compute the maximum over the entire computational domain, just the specified volume, andthis volume must lie within a single mesh. Other STATISTICS are discussed below. Note that some areappropriate for gas phase output quantities, some for solid phase, and some for both.

For solid phase output quantities, like heat fluxes and surface temperatures, the specification of aSURF_ID along with the appropriate statistic limits the calculation to only those surfaces. You can fur-ther limit the search by using the sextuplet of coordinates XB to force FDS to only compute statistics forsurface cells within the given volume. Be careful to account for the fact that the solid surface might shift toconform to the underlying numerical grid. Also, be careful not to specify a volume that extends beyond asingle mesh. Note that you do not (and should not) specify an orientation via the parameter IOR when usinga spatial statistic. IOR is only needed to find a specific point on the solid surface.

Use the STATISTICS feature with caution because it demands that FDS evaluate the given QUANTITY in allgas or solid phase cells.

Minimum or Maximum Value

For a given gas phase scalar output quantity defined at the center of each grid cell, φi jk, STATISTICS=’MIN’or STATISTICS=’MAX’ computes the minimum or maximum value, respectively

mini jk

φi jk ; maxi jk

φi jk (14.21)

over the cells that are included in the specified volume bounded by XB. Note that this statistic is only ap-propriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinateparameters, XB, all of which must be contained within the same mesh.

Average Value

For a given gas phase scalar output quantity defined at the center of each grid cell, φi jk, STATISTICS=’MEAN’computes the average value,

1N ∑

i jkφi jk (14.22)

over the cells that are included in the specified volume bounded by XB. Note that this statistic is only ap-propriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinateparameters, XB, all of which must be contained within the same mesh.

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Volume-Weighted Mean

For a given gas phase output quantity, φ(x,y,z), STATISTICS=’VOLUME MEAN’ produces the discrete ana-log of

1V

∫φ(x,y,z) dxdydz (14.23)

which is very similar to ’MEAN’, but it weights the values according to the relative size of the mesh cell.Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volumeto sum over via the coordinate parameters, XB, all of which must be contained within the same mesh.

Mass-Weighted Mean

For a given gas phase output quantity, φ(x,y,z), STATISTICS=’MASS MEAN’ produces the discrete analogof ∫

ρ(x,y,z)φ(x,y,z) dxdydz∫ρ dxdydz

(14.24)

which is similar to ’VOLUME MEAN’, but it weights the values according to the relative mass of the meshcell. Note that this statistic is only appropriate for gas phase quantities. Note also that you must specifya volume to sum over via the coordinate parameters, XB, all of which must be contained within the samemesh.

Volume Integral

For a given gas phase output quantity, φ(x,y,z), STATISTICS=’VOLUME INTEGRAL’ produces the discreteanalog of ∫

φ(x,y,z) dxdydz (14.25)

Note that this statistic is only appropriate for gas phase quantities, in particular those whose units involvem−3. For example, heat release rate per unit volume is an appropriate output quantity. Note also that youmust specify a volume to sum over via the coordinate parameters, XB, all of which must be contained withinthe same mesh.

Mass Integral

For a given gas phase output quantity, φ(x,y,z), STATISTICS=’MASS INTEGRAL’ produces the discreteanalog of ∫

ρ(x,y,z)φ(x,y,z) dxdydz (14.26)

Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volumeto sum over via the coordinate parameters, XB, all of which must be contained within the same mesh.

Area Integral

For a given gas phase output quantity, φ(x,y,z), STATISTICS=’AREA INTEGRAL’ produces the discreteanalog of ∫

φ(x,y,z) dA (14.27)

where dA depends on the coordinates you specify for XB. Note that this statistic is only appropriate for gasphase quantities, in particular those whose units involve m−2. For example, the quantity ’MASS FLUX X’

161

along with SPEC_ID=’my gas’ is an appropriate output quantity if you want to know the mass flux of thegas species that you have named ’my gas’ through an area normal to the x direction. Note also that youmust specify an area to sum over via the coordinate parameters, XB, all of which must be contained withinthe same mesh.

Surface Integral

For a given solid phase output quantity, φ, STATISTICS=’SURFACE INTEGRAL’ produces the discreteanalog of ∫

φ dA (14.28)

Note that this statistic is only appropriate for solid phase quantities, in particular those whose units involvem−2. For example, the various heat and mass fluxes are appropriate output quantities.

Mass and Energy Flows

The net flow of mass and energy into or out of compartments can be useful for many applications. Thereare several outputs that address these. All are prescribed via the device (DEVC) namelist group only. Forexample:

&DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0, QUANTITY='MASS FLOW', ID='whatever' /

outputs the net integrated mass flux through the given planar area, oriented in the positive z direction, in thiscase. The three flows – ’VOLUME FLOW’, ’MASS FLOW’, and ’HEAT FLOW’ are defined:

V =∫

u ·dS

m =∫

ρu ·dS

q =∫

cpρ(T −T∞)u ·dS

The addition of a + or - to the QUANTITY names yields the integral of the flow in the positive or negativedirection only. In other words, if you want to know the mass flow out of a compartment, use ’MASS FLOW

+’ or ’MASS FLOW -’, depending on the orientation of the door.

14.3.11 Temporally-Integrated Outputs

In addition to the spatial statistics, a time integral of an DEVC output can be computed by specifyingSTATISTICS = ’TIME INTEGRAL’ on the DEVC line. This produces a discrete analog of∫ t

t0φ(τ) dτ (14.29)

Note that the spatial and time integrals can not be used simultaneously.

14.3.12 Wind and the Pressure Coefficient

In the field of wind engineering, a commonly used quantity is known as the PRESSURE_COEFFICIENT:

Cp =p− p∞

12 ρ∞U2

(14.30)

p∞ is the ambient, or “free stream” pressure, and ρ∞ is the ambient density. The parameter U is the free-stream wind speed, given as CHARACTERISTIC_VELOCITY on the PROP line

162

&BNDF QUANTITY='PRESSURE COEFFICIENT', PROP_ID='whatever' /&DEVC ID='pressure tap', XYZ=..., IOR=2, QUANTITY='PRESSURE COEFFICIENT', PROP_ID='whatever' /&PROP ID='whatever', CHARACTERISTIC_VELOCITY=3.4 /

Thus, you can either paint values of Cp at all surface points, or create a single time history of Cp using asingle device at a single point.

14.3.13 Dry Volume and Mass Fractions

During actual experiments, species such as CO and CO2 are typically measured “dry”; that is, the watervapor is removed from the gas sample prior to analysis. For easier comparison of FDS predictions with mea-sured data, you can specify the logical parameter DRY on a DEVC line that reports the ’MASS FRACTION’

or ’VOLUME FRACTION’ of a species when using the mixture fraction combustion model. For example, thefirst line reports the actual volume fraction of CO, and the second line reports the output of a gas analyzerin a typical experiment.

&DEVC ID='wet CO', XYZ=..., QUANTITY='VOLUME FRACTION', SPEC_ID='carbon monoxide' /&DEVC ID='dry CO', XYZ=..., QUANTITY='VOLUME FRACTION', SPEC_ID='carbon monoxide', DRY=.TRUE. /

14.3.14 Gas Velocity

The gas velocity components, u, v, and w, can be output as slice (SLCF), point device (DEVC), isosurface(ISOF), or Plot3D quantities using the names ’U-VELOCITY’, ’V-VELOCITY’, and ’W-VELOCITY’. Thetotal velocity is specified as just ’VELOCITY’. Normally, the velocity is always positive, but you can usethe parameter VELO_INDEX with a value of 1, 2 or 3 to indicate that the velocity ought to have the same signas u, v, or w, respectively. This is handy if you are comparing velocity predictions with measurements. ForPlot3D files, add PLOT3D_VELO_INDEX(N)=... to the DUMP line, where N refers to the Plot3D quantity1, 2, 3, 4 or 5.

14.4 Extracting Numbers from the Output Data Files

Often it is desired to present results of calculations in some form other than those offered by Smokeview.In this case, there is a short Fortran 90 program called fds2ascii.f90, with a PC compiled version calledfds2ascii.exe. To run the program, just type

fds2ascii

at the command prompt. You will be asked a series of questions about which type of output file to pro-cess, what time interval to time average the data, and so forth. A single file is produced with the nameCHID_fds2ascii.csv. A typical command line session looks like this:

>> fds2asciiEnter Job ID string (CHID):

bucket_testWhat type of file to parse?PL3D file? Enter 1SLCF file? Enter 2BNDF file? Enter 3

3Enter Sampling Factor for Data?(1 for all data, 2 for every other point, etc.)

1

163

Limit the domain size? (y or n)y

Enter min/max x, y and z-5 5 -5 5 0 1

1 MESH 1, WALL TEMPERATUREEnter starting and ending time for averaging (s)

35 36Enter orientation: (plus or minus 1, 2 or 3)

3Enter number of variables

1Enter boundary file index for variable 1

1Enter output file name:

bucket_test_fds2ascii.csvWriting to file... bucket_test_fds2ascii.csv

These commands tell fds2ascii that you want to convert (binary) boundary file data into a text file. You wantto sample every data point within the specified volume, you want only those surfaces that point upwards (+3orientation), you only want 1 variable (only one is listed anyway and its index is 1 – that is just the numberused to list the available files). The data will be time-averaged, and it will be output to a file listed at the endof the session.

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14.5 Summary of Frequently-Used Output Quantities

Table 14.1, spread over the following pages, summarizes the various Output Quantities. The column “FileType” lists the allowed output files for the quantities. “B” is for Boundary (BNDF), “D” is for Device (DEVC),“I” is for Iso-surface (ISOF), “P” is for Plot3D, “PA” for PArticle (PART), “S” is for Slice (SLCF). Be carefulwhen specifying complicated quantities for Iso-surface or Plot3D files, as it requires computation in everygas phase cell.

For those output quantities that require a species name via SPEC_ID, the species implicitly defined whendoing a mixture fraction calculation are as follows:

fueloxygennitrogenwater vaporcarbon dioxidecarbon monoxidehydrogensootother

As an example of how to use the species names, suppose you want to calculate the integrated mass flux ofcarbon monoxide through a horizontal plane, like the total amount entrained in a fire plume. Use a “device”as follows

&DEVC ID='CO_flow', XB=-5,5,-5,5,2,2, QUANTITY='MASS FLUX Z', SPEC_ID='carbon monoxide',STATISTICS='AREA INTEGRAL' /

Here, the ID is just a label in the output file. The use of lower case letters for the SPEC_ID indicates thatyou want to record the mass flow of the CO that is implicitly defined by the mixture fraction variables. Thisis the default combustion model in FDS. If you have defined a species explicitly via a SPEC line, you canspecify that instead.

The format of certain output quantities changed, starting with FDS version 5.2, but the older names origi-nating with version 5.0 will still be accepted. The new convention is that when an output quantity is relatedto a particular gas species or particle type, then you must specify the appropriate SPEC_ID or PART_ID onthe same input line. Also note that the use of underscores in output quantity names has been eliminated –just remember that all output quantity names ought to be in single quotes.

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Table 14.1: Summary of frequently used output quantities.

QUANTITY Symbol Units File TypeABSORPTION COEFFICIENT κ (Section 11.3) 1/m D,I,P,SACTUATED SPRINKLERS Number of actuated sprinklers DADIABATIC SURFACE TEMPERATURE See Section 14.3.7 C B,DAMPUA∗∗ See Section 14.3.6 kg/m2 B,DASPIRATION See Section 13.3.6 % DAVERAGE SPECIFIC HEAT cp kJ/kg/K D,I,P,SBACK WALL TEMPERATURE See Section 14.2.2 C B,DBURNING RATE m′′f kg/m2/s B,DCHAMBER OBSCURATION See Section 13.3.4 %/m DCONDUCTIVITY k W/m/k D,I,P,SCONTROL See Section 13.5 DCONVECTIVE HEAT FLUX q′′c (Section 14.3.5) kW/m2 B,DCPUA∗∗ See Section 14.3.6 kW/m2 B,DCPU TIME Elapsed CPU time s DDENSITY ρ or ρYα with SPEC_ID kg/m3 D,I,P,SDIVERGENCE ∇ ·u s−1 D,I,P,SDROPLET FLUX X∗∗ See Section 14.3.6 kg/m2/s P,SDROPLET FLUX Y∗∗ See Section 14.3.6 kg/m2/s P,SDROPLET FLUX Z∗∗ See Section 14.3.6 kg/m2/s P,SDROPLET DIAMETER 2rd µm PADROPLET VELOCITY |ud | m/s PADROPLET TEMPERATURE Td

C PADROPLET MASS md kg PADROPLET AGE td s PAENTHALPY ∑ρYα

∫ T0 cp,α(T ′)dT ′ kJ/m3 D,I,P,S

FED See Section 14.3.9 DGAUGE HEAT FLUX See Section 14.3.5 kW/m2 B,DH H = |u|2/2+ p/ρ0 (m/s)2 D,I,P,SHEAT FLOW See Section 14.3.10 kW DNET HEAT FLUX See Section 14.3.5 kW/m2 B,DHRR

∫q′′′ dV kW D

HRRPUV q′′′ kW/m3 D,I,P,SINCIDENT HEAT FLUX See Section 14.3.5 kW/m2 B,DINSIDE WALL TEMPERATURE See Section 14.2.2 C DITERATION Number of time steps DLAYER HEIGHT See Section 14.3.3 m DLINK TEMPERATURE See Section 13.3.3 C DLOWER TEMPERATURE See Section 14.3.3 C DMASS FLOW See Section 14.3.10 kg/s DMASS FLUX∗ Mass flux at solid surface kg/m2/s B,DMASS FLUX X∗ ρuYα kg/m2/s D,I,P,SMASS FLUX Y∗ ρvYα kg/m2/s D,I,P,SMASS FLUX Z∗ ρwYα kg/m2/s D,I,P,S

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Table 14.1: Summary of frequently used output quantities (continued).

QUANTITY Symbol Units File TypeMASS FRACTION∗ Yα kg/kg D,I,P,SMIXTURE FRACTION Z kg/kg D,I,P,SMPUA∗∗ See Section 14.3.6 kg/m2 B,DMPUV∗∗ See Section 14.3.6 kg/m3 D,P,SNORMAL VELOCITY Wall normal velocity m/s D,BOPEN NOZZLES Number of open nozzles DOPTICAL DENSITY K/2.3 (Section 14.3.2) 1/m D,I,P,SEXTINCTION COEFFICIENT K (Section 14.3.2) 1/m D,I,P,SPATH OBSCURATION See Section 13.3.5 % DPRESSURE Perturbation pressure, p Pa D,I,P,SPRESSURE COEFFICIENT Cp (Section 14.3.12) B,DPRESSURE ZONE See Section 9.6 D,SRADIATIVE HEAT FLUX See Section 14.3.5 kW/m2 B,DRADIATIVE HEAT FLUX GAS See Section 14.3.5 kW/m2 DRADIOMETER See Section 14.3.5 kW/m2 B,DRELATIVE HUMIDITY Relative humidity % D,I,P,SSOOT VOLUME FRACTION ρYs(Z)/ρs mol/mol D,I,P,SSPECIFIC ENTHALPY ∑Yα

∫ T0 cp,α(T ′)dT ′ kJ/kg D,I,P,S

SPECIFIC HEAT cp kJ/kg/K D,I,P,SSPRINKLER LINK TEMPERATURE See Section 13.3.1 C DSOLID DENSITY See Section 14.3.8 kg/m3 DSURFACE DENSITY See Section 14.3.8 kg/m2 B,DTEMPERATURE T (Section 14.3.4) C D,I,P,STHERMOCOUPLE TTC (Section 14.3.4) C DTIME t (Section 13.1) s DTIME STEP δt, Numerical time step s DU-VELOCITY u m/s D,I,P,SV-VELOCITY v m/s D,I,P,SW-VELOCITY w m/s D,I,P,SUPPER TEMPERATURE See Section 14.3.3 C DVELOCITY∗∗∗

√u2 + v2 +w2 m/s D,I,P,S

VISCOSITY µ kg/m/s D,I,P,SVISIBILITY S = C/K (Section 14.3.2) m D,I,P,SVOLUME FLOW See Section 14.3.10 m3/s DVOLUME FRACTION∗∗∗∗ Xα mol/mol D,I,P,SWALL CLOCK TIME Elapsed wall clock time s DWALL TEMPERATURE Tw

C B,DWALL THICKNESS See Section 14.3.8 m B,D

∗ Quantity requires the specification of a gas species using SPEC_ID.∗∗ Quantity requires the specification of a particle name using PART_ID.∗∗∗ Add VELO_INDEX=1 to the input line if you want to multiply the velocity by the sign of u.

167

Use the indices 2 and 3 for v and w, respectively.∗∗∗∗ Quantity requires the specification of a gas species using SPEC_ID.

Do not use for MIXTURE FRACTION.

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14.6 Summary of Infrequently-Used Output Quantities

Table 14.2 below lists some less often used output quantities. These are mainly used for diagnostic purposes.Explanations for most can be found in Volume 1 of the FDS Technical Reference Guide [1].

Table 14.2: Summary of infrequently used output quantities.

QUANTITY Symbol Units File TypeADD Average Droplet Diameter µm D,I,P,SADT Average Droplet Temperature C D,I,P,SDROPLET PHASE Orientation of droplet PAEMISSIVITY Surface emissivity (usually constant) B,DF_X, F_Y, F_Z Momentum terms, Fx, Fy, Fz m/s2 D,I,P,SGAS TEMPERATURE Gas Temperature near wall C B,DHEAT TRANSFER COEFFICIENT Convective heat transfer kW/m2/K B,DHP H ′ (pressure correction) (m/s)2 D,I,P,SHRRPUL

∫q′′′ dxdy kW/m D

INTEGRATED INTENSITY U =∫

I ds kW/m2 D,I,P,SKINETIC ENERGY (u2 + v2 +w2)/2 (m/s)2 D,I,P,SMAXIMUM VELOCITY ERROR See Section 6.6 DMIXING TIME Combustion mixing time s D,I,P,SPDPA Droplet diagnostics DPRESSURE ITERATIONS No. pressure iterations DRADIATION LOSS ∇ ·q′′r kW/m3 D,I,P,SSTRAIN RATE X ∂w/∂y+∂v/∂z 1/s D,I,P,SSTRAIN RATE Y ∂u/∂z+∂w/∂x 1/s D,I,P,SSTRAIN RATE Z ∂v/∂x+∂u/∂y 1/s D,I,P,SVORTICITY X ∂w/∂y−∂v/∂z 1/s D,I,P,SVORTICITY Y ∂u/∂z−∂w/∂x 1/s D,I,P,SVORTICITY Z ∂v/∂x−∂u/∂y 1/s D,I,P,SWATER RADIATION LOSS ∇ ·q′′r due to water droplets kW/m3 D,I,P,S

169

170

Chapter 15

Alphabetical List of Input Parameters

This Appendix lists all of the input parameters for FDS in separate tables grouped by Namelist, these tablesare in alphabetical order along with the parameters within them. This is intended to be used as a quickreference and does not replace reading the detailed description of the parameters in the main body of thisguide. See Table 5.1 for a cross-reference of relevant sections and the tables in this Appendix. The reasonfor this statement is that many of the listed parameters are mutually exclusive – specifying more than onecan cause the program to either fail or run in an unpredictable manner. Also, some of the parameters triggerthe code to work in a certain mode when specified. For example, specifying the thermal conductivity of asolid surface triggers the code to assume the material to be thermally-thick, mandating that other propertiesbe specified as well. Simply prescribing as many properties as possible from a handbook is bad practice.Only prescribe those parameters which are necessary to describe the desired scenario.

171

15.1 BNDF (Boundary File Parameters)

Table 15.1: For more information see Section 14.2.4.

BNDF (Boundary File Parameters)CELL_CENTERED Logical Do not do corner averaging .FALSE.

FYI Character Comment String (has no effect)PART_ID Character Particle identifier (if needed)PROP_ID Character Property identifier (if needed)RECOUNT_DRIP Logical Adds mass to AWMPUA for each drop impact .FALSE.QUANTITY Character Quantity to visualizeSPEC_ID Character Species identifier (if needed)

15.2 CLIP (MIN/MAX Clipping Parameters)

Table 15.2: For more information see Section 6.7.

CLIP (Specified Upper and Lower Limits)FYI Character Comment String (has no effect)MAXIMUM_DENSITY Real Maximum Gas Density kg/m3

MAXIMUM_MASS_FRACTION Real Array Maximum Gas Mass Fraction kg/kgMAXIMUM_TEMPERATURE Real Maximum Gas Temperature CMINIMUM_DENSITY Real Minimum Gas Density kg/m3

MINIMUM_MASS_FRACTION Real Array Minimum Gas Mass Fraction kg/kgMINIMUM_TEMPERATURE Real Maximum Gas Temperature C

15.3 CTRL (Control Function Parameters)

Table 15.3: For more information see Section 13.5.

CTRL (Control Function Parameters)DELAY Real Time delay s 0.FUNCTION_TYPE Character Type of control functionID Character IDentifierINITIAL_STATE Logical Initial state of control function .FALSE.

INPUT_ID Char. Array DEVC and/or CTRL input IDsLATCH Logical Control function changes state only once .TRUE.

N Integer Number of .TRUE. INPUTs 1ON_BOUND Character Active edge of a DEADBAND LOWER

RAMP_ID Character ID for a CUSTOM ramp controllerSETPOINT(2) Real Lower and upper bound of a DEADBAND

172

15.4 DEVC (Device Parameters)

Table 15.4: For more information see Section 13.1.

DEVC (Device Parameters)BYPASS_FLOWRATE Real Aspiration smoke detector parameter kg/s 0CTRL_ID Character Associated CTRL lineDELAY Real Transport time for an aspiration detector s 0DEVC_ID Character Associated DEVC line for aspiration detectorDEPTH Real Depth into surface for internal wall temp m 0FLOWRATE Real Suction flowrate for an aspiration detector kg/s 0FYI Character Comment String (has no effect)IOR Integer Index of Orientation (±1,±2,±3)ID Character Identifying label for outputINITIAL_STATE Logical Initial state of device .FALSE.

LATCH Logical Device cannot change state multiple times .TRUE.

MATL_ID Character Material identifier (if needed)ORIENTATION Real Triplet Direction vector 0,0,-1PART_ID Character Particle identifier (if needed)PROP_ID Character Associated PROPerty lineQUANTITY Character Name of Quantity to outputROTATION Real Triplet Rotation Angle deg 0SETPOINT Real Value at which device changes stateSPEC_ID Character Species identifier (if needed)STATISTICS Character See Section 14.3.10SURF_ID Character See Section 14.3.10TIME_AVERAGED Logical See Section 13.2 .TRUE.

TRIP_DIRECTION Integer Sign of derivative at first state change 1VELO_INDEX Integer See Section 14.3.14 0XB(6) Real Sextuplet Coordinates of non-point measurement mXYZ Real Triplet Physical coordinates m

15.5 DUMP (Output Parameters)

Table 15.5: For more information see Section 14.1.

DUMP (Output Parameters)CHECK_VOLUME_FLOW Logical See Section ?? .FALSE.

COLUMN_DUMP_LIMIT Logical Limit text output to 255 columns .TRUE.

CTRL_COLUMN_LIMIT Integer Number of columns in CTRL file 254DEVC_COLUMN_LIMIT Integer Number of columns in DEVC file 254DT_BNDF Real Boundary dump interval s 2∆t/NFRAMESDT_CTRL Real Control status dump interval s ∆t/NFRAMESDT_DEVC Real Device output dump interval s ∆t/NFRAMES

173

Table 15.5: Continued

DUMP (Output Parameters)DT_FLUSH Real See Section 14.1 s ∆t/NFRAMESDT_HRR Real Heat release dump interval s ∆t/NFRAMESDT_ISOF Real Iso-surface dump interval s ∆t/NFRAMESDT_MASS Real Mass diagnostic dump interval s ∆t/NFRAMESDT_PART Real Particle dump interval s ∆t/NFRAMESDT_PL3D Real PLOT3D dump interval s ∆t/5DT_PROF Real Profile dump interval s ∆t/NFRAMESDT_RESTART Real Restart core dump interval s 1000000.DT_SLCF Real Slice dump interval s ∆t/NFRAMESFLUSH_FILE_BUFFERS Logical See Section 14.1 .TRUE.

MASS_FILE Logical Flag for species MASS file .FALSE.

MAXIMUM_DROPLETS Integer Max particles per mesh 500000NFRAMES Integer Number of Frames of output data 1000PLOT3D_QUANTITY(5) Char. Quint See Section 14.2.6PLOT3D_SPEC_ID(5) Char. Quint See Section 14.2.6PLOT3D_VELO_INDEX Integer Quint See Section 14.3.14 0SMOKE3D Logical Flag for 3D Smoke Visualization .TRUE.

SMOKE3D_QUANTITY Character See Section 14.2.7SMOKE3D_SPEC_ID Character See Section 14.2.7STATE_FILE Logical Flag for state relation file .FALSE.

STATUS_FILES Logical Flag for status (notready) file .FALSE.

WRITE_XYZ Logical Flag for writing PLOT3D .xyz file .FALSE.

∆t=T_END-T_BEGIN

15.6 HEAD (Header Parameters)

Table 15.6: For more information see Section 6.1.

HEAD (Header Parameters)CHID Character Job Identification String ’output’

FYI Character Comment String (has no effect)TITLE Character Title for job

15.7 HOLE (Obstruction Cutout Parameters)

Table 15.7: For more information see Section 7.3.

HOLE (Obstruction Cutout Parameters)COLOR Character Color name of obstruction color

174

Table 15.7: Continued

HOLE (Obstruction Cutout Parameters)CTRL_ID Character ID of ConTRoL to control hole’s existenceDEVC_ID Character ID of DEViCe to control hole’s existenceFYI Character Comment String (has no effect)RGB(3) Integer Triplet Color indices (0 - 255) for resulting obstruction(s)TRANSPARENCY Real Transparency of obstructionXB(6) Real Sextuplet Physical coordinates m

15.8 INIT (Initial Conditions)

Table 15.8: For more information see Section 6.5.

INIT (Initial Conditions)DENSITY Real Initial value of density kg/m3 AmbientMASS_FRACTION(II) Real Array Initial value of species II kg/kg AmbientMASS_PER_TIME Real See Section 12.2.3 kg/sMASS_PER_VOLUME Real See Section 12.2.3 kg/m3 1NUMBER_INITIAL_DROPLETS Integer See Section 12.2.3 0PART_ID Character See Section 12.2.3TEMPERATURE Real Initial value of temperature C TMPA

XB(6) Real Sextuplet Coordinates m

15.9 ISOF (Isosurface Parameters)

Table 15.9: For more information see Section 14.2.5.

ISOF (Isosurface Parameters)FYI Character Comment String (has no effect)QUANTITY Character Quantity to visualizeSPEC_ID Character Species indicator (if needed)VALUE(I) Real Array Contour value(s)VELO_INDEX Integer See Section 14.3.14 0

15.10 MATL (Material Properties)

Table 15.10: For more information see Section 8.3.

MATL (Material Properties)A Real array Pre-exponential factors 1/s

175

Table 15.10: Continued

MATL (Material Properties)ABSORPTION_COEFFICIENT Real Absorption Coefficient 1/m 50000.BOILING_TEMPERATURE Real Boiling temperature C 5000.CONDUCTIVITY Real Thermal conductivity W/m/K 0.CONDUCTIVITY_RAMP Character Ramp ID for conductivityDENSITY Real Solid mass per unit volume kg/m3 0.E Real array Activation energies kJ/kmolEMISSIVITY Real Emissivity 0.9FYI Character Comment String (has no effect)HEATING_RATE Real array See Section 8.4.4 C/min 5.HEAT_OF_COMBUSTION(:,II) Real array Heats of combustion for species II kJ/kg 0.HEAT_OF_REACTION(:) Real array Heats of reaction kJ/kg 0.INITIAL_VAPOR_FLUX Real Initial evaporation rate m2/(sm2) 0.0005ID Character IDentifierTHRESHOLD_TEMPERATURE Real array See Section 8.4.4 C -273.15N_REACTIONS Character Number of Reactions 0N_S Real array See Section 8.4.4 1.N_T Real array See Section 8.4.4 0.NU_FUEL Real array Fuel gas yields kg/kg 0.NU_GAS(:,II) Real array Yields of species II kg/kg 0.NU_RESIDUE Real array Residue yields kg/kg 0.NU_WATER Real aray Water vapor yields kg/kg 0.PYROLYSIS_RANGE Real array See Section 8.4.4 C 80.REFERENCE_TEMPERATURE Real array See Section 8.4.4 CRESIDUE Character IDs of residue MATLSPECIFIC_HEAT Real Specific heat kJ/kg/K 0.SPECIFIC_HEAT_RAMP Character Ramp ID for specific heat

15.11 MESH (Mesh Parameters)

Table 15.11: For more information see Section 6.3.

MESH (Mesh Parameters)COLOR Character Mesh Line Color ’BLACK’

CYLINDRICAL Logical 2-D Axi-symmetric calculation .FALSE.

ID Character MESH IDentifierIJK Integer Triplet No. cells in x, y, and z directions 10FYI Character Comment String (has no effect)MPI_PROCESS Integer See Section 6.3.3RGB Integer Triplet Color indices (0-255) 0,0,0SYNCHRONIZE Logical Sync. time steps of multiple meshes .TRUE.

XB(6) Real Sextuplet Min/Max Coordinates of the MESH m 0,1,0,1,0,1

176

15.12 MISC (Miscellaneous Parameters)

Table 15.12: For more information see Section 6.4.

MISC (Miscellaneous Parameters)ALLOW_UNDERSIDE_DROPLETS Logical See Section 12.2 .FALSE.

ASSUMED_GAS_TEMPERATURE Real See Section 8.5BACKGROUND_SPECIES Character See Section 11.2 ’AIR’

BAROCLINIC Logical Baroclinic torque correction .TRUE.

BNDF_DEFAULT Logical See Section 14.2.4 .TRUE.

CFL_MAX Real See Section 6.4.7 1.0CFL_MIN Real See Section 6.4.7 0.8C_FORCED Real See Section 8.2.2 0.037C_HORIZONTAL Real See Section 8.2.2 1.52C_VERTICAL Real See Section 8.2.2 1.31CSMAG Real Smagorinsky constant 0.20CONDUCTIVITY Real See Section 11.2 W/m/KCO_PRODUCTION Logical See Section 11.1.2 .FALSE.

DNS Logical Direct Numerical Simulation .FALSE.

FYI Character Comment String (has no effect)GROUND_LEVEL Real See Section 9.4 m 0.GVEC Real triplet Gravity vector m/s2 0,0,-9.81H_EDDY Logical See Section 8.2.2 .FALSE.

HUMIDITY Real Relative Humidity % 40.ISOTHERMAL Logical Isothermal calculation .FALSE.

LAPSE_RATE Real See Section 9.4 C/m 0LES Logical Large Eddy Simulation .TRUE.

MW Real Molecular Weight (Section 11.2) g/molNOISE Logical Toggle initial noise on and off .TRUE.

PR Real Prandtl number (LES only) 0.5P_INF Real Ambient pressure Pa 101325POROUS_FLOOR Logical See Section 13.3.1 .TRUE.

RADIATION Logical Radiation calculation flag .TRUE.

RAMP_GX Character Time function, x comp. of gravityRAMP_GY Character Time function, y comp. of gravityRAMP_GZ Character Time function, z comp. of gravityRESTART Logical Restart previous calculation .FALSE.

RESTART_CHID Character Restart file CHID CHID

SC Real Schmidt number (LES only) 0.5SOLID_PHASE_ONLY Logical See Section 8.5 .FALSE.

STRATIFICATION Logical See Section 9.4 .TRUE.

SUPPRESSION Logical See Section 11.1.2 .TRUE.

SURF_DEFAULT Character Default SURFace type ’INERT’

TEXTURE_ORIGIN(3) Char. Triplet See Section 7.5.1 m (0.,0.,0.)THICKEN_OBSTRUCTIONS Logical See Section 7.2 .FALSE.

TMPA Real Ambient Temperature C 20.

177

Table 15.12: Continued

MISC (Miscellaneous Parameters)U0,V0,W0 Reals Prevailing velocity field m/s 0.VISCOSITY Real See Section 11.2 kg/m/sVN_MAX Real See Section 6.4.7 1.0VN_MIN Real See Section 6.4.7 0.8

15.13 MULT (Multiplier Function Parameters)

Table 15.13: For more information see Section 7.2.2.

MULT (Multiplier Function Parameters)DXB Real Sextuplet Spacing for all 6 coordinates m 0.DX Real Spacing in the x direction m 0.DY Real Spacing in the y direction m 0.DZ Real Spacing in the z direction m 0.DX0 Real Translation in the x direction m 0.DY0 Real Translation in the y direction m 0.DZ0 Real Translation in the z direction m 0.ID Character IDentifierI_LOWER Integer Lower array bound, x direction 0I_UPPER Integer Upper array bound, x direction 0J_LOWER Integer Lower array bound, y direction 0J_UPPER Integer Upper array bound, y direction 0K_LOWER Integer Lower array bound, z direction 0K_UPPER Integer Upper array bound, z direction 0N_LOWER Integer Lower sequence bound 0N_UPPER Integer Upper sequence bound 0

15.14 OBST (Obstruction Parameters)

Table 15.14: For more information see Section 7.2.

OBST (Obstruction Parameters)ALLOW_VENT Logical Allow vents on obstruction .TRUE.

BNDF_FACE(-3:3) Logical Array See Section 14.2.4 .TRUE.

BNDF_OBST Logical See Section 14.2.4 .TRUE.

BULK_DENSITY Real See Section 8.4.6 1COLOR Character Color name of obstruction colorCTRL_ID Character ID of Controlling ConTRoL

DEVC_ID Character ID of Controlling DEViCe

FYI Character Comment String (has no effect)

178

Table 15.14: Continued

OBST (Obstruction Parameters)OUTLINE Logical Draw as Outline .FALSE.

PERMIT_HOLE Logical Allow a Hole .TRUE.

REMOVABLE Logical Allow obstruction to be removed .TRUE.

RGB(3) Integer Triplet Color indices (0 - 255)SAWTOOTH Logical See Section 7.2.3 .TRUE.

SURF_ID Character Associated SurfaceSURF_IDS(3) Character Triplet Associated Surfaces (top,side,bot.)SURF_ID6(6) Character Sextuplet Associated Surfaces (like XB)THICKEN Logical Force at least one cell thick .FALSE.

TEXTURE_ORIGIN(3) Real Triplet See Section 7.5.1 m (0.,0.,0.)TRANSPARENCY Real Transparency indicator 1XB(6) Real Sextuplet Min/Max Physical coordinates m

15.15 PART (Lagrangian Particles/Droplets)

Table 15.15: For more information see Section 12.

PART (Lagrangian Particles/Droplets)AGE Real Droplet lifetime s 100000.COLOR Character Default color of droplets ’BLACK’

CTRL_ID Character Name of controllerDENSITY Real Droplet density kg/m3 1000.DEVC_ID Character Name of controlling deviceDIAMETER Real Median Volumetric Diameter µm 500.DRAG_LAW Character Geometry for drag correlation ’SPHERE’

EVAPORATE Logical Assume liquid evaporation .TRUE.

FYI Character Comment StringFUEL Logical Liquid Fuel .FALSE.

GAMMA_D Real Size distribution parameter 2.4HEAT_OF_COMBUSTION Real Heat of Combustion kJ/kgHEAT_OF_VAPORIZATION Real Latent Heat of Vaporization kJ/kgH_V_REFERENCE_TEMPERATURE Real Temperature for Heat of Vaporization CHORIZONTAL_VELOCITY Real Droplet speed, horizontal m/s 0.2ID Character IdentifierINITIAL_TEMPERATURE Real Initial Temperature C TMPA

MASSLESS Logical Massless tracers .FALSE.

MAXIMUM_DIAMETER Real Break-up bound µm ∞

MINIMUM_DIAMETER Real Evaporation bound µm 20.MELTING_TEMPERATURE Real Melting Temperature CMONODISPERSE Logical Uniform droplet size .FALSE.

PROP_ID Character Name of Property lineQUANTITIES(10) Character Quantities for coloring

179

Table 15.15: Continued

PART (Lagrangian Particles/Droplets)RGB(3) Integers Color indices (0-255)SAMPLING_FACTOR Integer Filter for output file 1SIGMA_D Real Size distribution parameterSPEC_ID Character Name of gas speciesSPECIFIC_HEAT Real Droplet specific heat kJ/kg/KSTATIC Logical Stationary Particles .FALSE.

USER_DRAG_COEFFICIENT Real Constant drag coefficient -1.VERTICAL_VELOCITY Real Droplet speed, vertical m/s 0.5WATER Logical Water Droplet .FALSE.

15.16 PRES (Pressure Solver Parameters)

Table 15.16: For more information see Section 6.6.

PRES (Pressure Solver Parameters)MAX_PRESSURE_ITERATIONS Integer See Section 6.6 10000VELOCITY_TOLERANCE Real See Section 6.6 m/s

15.17 PROF (Wall Profile Parameters)

Table 15.17: For more information see Section 14.2.2.

PROF (Wall Profile Parameters)IOR Real Orientation of wall surfaceID Character IdentifierFYI Character Comment String (has no effect)QUANTITY Character Name of output quantityXYZ Real Triplet Coordinates of wall surface m

15.18 PROP (Device Properties)

Table 15.18: For more information see Section 13.3.

PROP (Device Properties)ACTIVATION_TEMPERATURE Real Threshold link temperature C 74ACTIVATION_OBSCURATION Real Threshold value of obscuration %/m 3.28ALPHA_C Real Smoke detector parameter 1.8ALPHA_E Real Smoke detector parameter 0.0

180

Table 15.18: Continued

PROP (Device Properties)BETA_C Real Smoke detector parameter 1.0BETA_E Real Smoke detector parameter 1.0BEAD_DIAMETER Real Diameter of TC bead m 0.001BEAD_EMISSIVITY Real Emissivity of TC bead 0.85CABLE_DIAMETER Real See Section 13.3.7 m 0.02CABLE_FAILURE_TEMPERATURE Real See Section 13.3.7 C 400CABLE_JACKET_THICKNESS Real See Section 13.3.7 m 0.002CABLE_MASS_PER_LENGTH Real See Section 13.3.7 kg/m 0.5C_FACTOR Real Sprinkler activation parameter 0.CHARACTERISTIC_VELOCITY Real See Section 14.3.12 m/s 1.0DROPLET_VELOCITY Real Initial droplet velocity m/s 0.0DROPLETS_PER_SECOND Integer Drops per second per head 5000DT_INSERT Real Time between insertions s 0.01FLOW_RATE Real Sprinkler or nozzle flow rate L/minFLOW_RAMP Character Time RAMP for flowFLOW_TAU Real Time constant for flow 0.0GAUGE_TEMPERATURE Real See Section 14.3.5 C TMPA

ID Character IDentifierINITIAL_TEMPERATURE Real Initial link temperature C TMPA

K_FACTOR Real Flow parameter L/min/atm1/2 1.LENGTH Real Smoke detector parameter 1.8OFFSET Real Droplet offset distance m 0.05OPERATING_PRESSURE Real Sprinkler pipe pressure atm 1.ORIFICE_DIAMETER Real Nozzle orifice diameter m 0.0PART_ID Character Name of associated PART linePDPA_END Real See Section 14.3.6 s T_END

PDPA_M Integer See Section 14.3.6 0PDPA_M Integer See Section 14.3.6 0PDPA_RADIUS Real See Section 14.3.6 m 0.PDPA_START Real See Section 14.3.6 s 0.PRESSURE_RAMP Character Pressure RAMP for sprinklersQUANTITY Character Name of associated outputRTI Real Response Time Index

√m s 100.

SMOKEVIEW_ID Char. Array Name(s) of drawn objectSMOKEVIEW_PARAMETERS Char. Array Misc. parameters for drawingSPEC_ID Character See Section 13.3.4SPRAY_ANGLE(2) Real Cone angles for water spray deg 60.,75.SPRAY_PATTERN_TABLE Character TABL for spray pattern

15.19 RADI (Radiation Parameters)

181

Table 15.19: For more information see Section 11.3.

RADI (Radiation Parameters)ANGLE_INCREMENT Integer Number of angles skipped per update 5CH4_BANDS Logical Include extra fuel bands .FALSE.

KAPPA0 Real Constant absorption coefficient 1/m 0NMIEANG Integer Number of polar angles 15NUMBER_RADIATION_ANGLES Integer Number of solid angles 104PATH_LENGTH Real Path length for radiation calc. mRADIATIVE_FRACTION Real Radiative Loss Fraction 0.35RADTMP Real Assumed radiative source temp. C 900TIME_STEP_INCREMENT Integer Number time steps skipped 3WIDE_BAND_MODEL Logical Non-gray gas assumption .FALSE.

15.20 RAMP (Ramp Function Parameters)

Table 15.20: For more information see Section 10.

RAMP (Ramp Function Parameters)DEVC_ID Character See Section 13.6F Real Function valueFYI Character Comment String (has no effect)ID Character IdentifierT Real Time (or Temperature) s (or C)X Real x-coordinate (gravity only) m

15.21 REAC (Reaction Parameters)

Table 15.21: For more information see Section 11.1.

REAC (Reaction Parameters)BOF Real Pre-exponential Factor (Finite Rate) cm3/mol/sC Real Carbon atoms in fuel 3C_EDC Real See Section 11.1.5 0.1CO_YIELD Real Fraction of CO from the fuel kg/kg 0CRITICAL_FLAME_TEMPERATURE Real Suppression criterion C 1427E Real Activation Energy (Finite Rate) kJ/kmolEDDY_DISSIPATION Logical See Section 11.1.5 .TRUE.

EPUMO2 Real Energy per Unit Mass Oxygen kJ/kg 13100FUEL Character Name of Fuel (Finite Rate) ETHYLENE

FYI Character Comment String (has no effect)H Real Hydrogen atoms in fuel 8H2_YIELD Real Fraction of H2 from the fuel kg/kg 0

182

Table 15.21: Continued

REAC (Reaction Parameters)HEAT_OF_COMBUSTION Real Energy per Unit Mass Fuel kJ/kgHRRPUA_SHEET Real See Section 11.1.5 kW/m2 0 (LES); 200 (DNS)HRRPUV_AVERAGE Real See Section 11.1.5 kW/m3 2500 (LES); 0 (DNS)ID Character IdentifierIDEAL Logical Adjust for minor product yields .FALSE.

MASS_EXTINCTION_COEFFICIENT Real Visibility parameter m2/kg 8700.MAXIMUM_VISIBILITY Real Visibility parameter m 30MW_OTHER Real Molecular Weight of OTHER g/mol 28N Real Nitrogen atoms in the fuel 0N_S(N) Real Arrhenius Exponents (Finite Rate)NU(N) Real Reaction stoichiometry (Finite Rate)O Real Oxygen atoms in the fuel 0OTHER Real Other atoms in the fuel 0OXIDIZER Character Name of Oxidizer (Finite Rate)SOOT_YIELD Real Fraction of soot from the fuel kg/kg 0.01SOOT_H_FRACTION Real Atom fraction of hydrogen in soot 0.1VISIBILITY_FACTOR Real Visibility parameter 3X_O2_LL Real Lower Oxygen Limit mol/mol 0.15Y_F_INLET Real Mass Frac. of Fuel in Burner kg/kg 1.0Y_F_LFL Real Lower Fuel limit (mass fraction) kg/kg 0.0Y_O2_INFTY Real Ambient oxygen mass fraction kg/kg 0.232428

15.22 SLCF (Slice File Parameters)

Table 15.22: For more information see Section 14.2.3.

SLCF (Slice File Parameters)CELL_CENTERED Logical Show raw data with no averaging .FALSE.

FYI Character Comment String (has no effect)MESH_NUMBER Integer Save only slices in this meshPART_ID Character Particle identifier (if needed)PBX Real x-plane to save slice filePBY Real y-plane to save slice filePBZ Real z-plane to save slice fileQUANTITY Character Name of Quantity to displaySPEC_ID Character Species identifier (if needed)VECTOR Logical Include flow vectors .FALSE.

VELO_INDEX Integer See Section 14.3.14 0XB(6) Real Sextuplet Min/Max coordinates of region to save m

183

15.23 SPEC (Species Parameters)

Table 15.23: For more information see Section 11.2.

SPEC (Species Parameters)ABSORBING Logical Absorbs radiation .FALSE.

CONDUCTIVITY Real Conductivity k W/m/KDIFFUSIVITY Real Diffusivity D m2/sEPSILONKLJ Real Leonard-Jones Parameter 0FORMULA Character Chemical formula for

Smokeview labelFYI Character Comment StringID Character Name of speciesMASS_EXTINCTION_COEFFICIENT Real See Section 13.3.4 0MASS_FRACTION_0 Real Initial mass fraction 0MW Real Molecular Weight g/mol 29.REFERENCE_TEMPERATURE Real Temperature for C 25.

Specific EnthalpySPECIFIC_ENTHALPY Real Specific Enthalpy kJ/kgSPECIFIC_HEAT Real Specific Heat kJ/kg/KSIGMALJ Real Leonard-Jones Parameter 0VISCOSITY Real Dynamic Viscosity mu kg/m/s

15.24 SURF (Surface Properties)

Table 15.24: For more information see Section 7.1.

SURF (Surface Properties)ADIABATIC Logical Adiabatic thermal BC .FALSE.

BACKING Character Back boundary condition ’VOID’

BURN_AWAY Logical See Section 8.4.6 .FALSE.

CELL_SIZE_FACTOR Real See Section 8.3.7 1.0COLOR Character Surface ColorCONVECTIVE_HEAT_FLUX Real Heat flux at surface kW/m2 0.DUCT_PATH Int. Pair Pressure Zones for fans 0,0E_COEFFICIENT Real Extinguishing coefficient m2/kg/s 0.EMISSIVITY Real Emissivity 0.9EXTERNAL_FLUX Real Heat flux to surface kW/m2 0.FREE_SLIP Logical See Section 9.5 .FALSE.

FYI Character Comment StringGEOMETRY Character Geometry type ’CARTESIAN’

H_FIXED Real See Section 8.2.2 W/m2/KHEAT_OF_VAPORIZATION Real For specified HRR only kJ/kg 0.HRRPUA Real HRR Per Unit Area kW/m2 0.

184

Table 15.24: Continued

SURF (Surface Properties)ID Character IDentifierIGNITION_TEMPERATURE Real Ignition temperature C 5000.LAYER_DIVIDE Real See Section 8.3.5 0.5 × n.of.layersLEAK_PATH Int. Pair Pressure Zones for leakageMASS_FLUX(I) Real Array For species I kg/m2 s 0.MASS_FLUX_TOTAL Real Total Mass Flux kg/m2 sMASS_FRACTION(I) Real Array For species IMATL_ID Char. Array (Layer,Component)MATL_MASS_FRACTION Real Array (Layer,Component)MAX_PRESSURE Real Max over-pressure for fan Pa 1.E12MLRPUA Real Mass loss rate per unit area kg/m2s 0.NET_HEAT_FLUX Real Net flux at surface kW/m2 0.NO_SLIP Logical See Section 9.5 .FALSE.

NPPC Integer Number of particles per cell 1PARTICLE_MASS_FLUX Real See Section 12.2.1 kg/m2 s 0.PART_ID Character Lagrangian Particle IDPOROUS Logical See Section 9.1 .FALSE.

PLE Real Atmospheric profile exp. 0.3PROFILE Character Name of velocity profileRAMP_MF(I) Character Ramp ID for species IRAMP_Q Character Ramp ID for HRRRAMP_T Character Ramp ID for temp.RAMP_V Character Ramp ID for velocityRGB(3) Int. Triplet Color indices (0-255) 255,204,102STRETCH_FACTOR Real See Section 8.3.7 2.0SURFACE_DENSITY Real See Section 8.4.6 kg/m2 0.0TAU_MF(I) Real Array Ramp time for species I s 1.TAU_Q Real Ramp time for HRR s 1.TAU_T Real Ramp time for temp. s 1.TAU_V Real Ramp time for velocity s 1.TEXTURE_HEIGHT Real Height of texture image m 1.TEXTURE_MAP Character Name of texture map fileTEXTURE_WIDTH Real Width of texture image m 1.THICKNESS(IL) Real Array Thickness of Layer IL m 0.TMP_BACK Real Back surface temperature BC C 20.TMP_FRONT Real Front surface temperature C 20.TMP_INNER Real Initial solid temperature C 20.TRANSPARENCY Real Transparency of obstruction 1VEL Real Normal velocity m/s 0.VEL_T Real Pair Tangential velocity comps. m/s 0.VOLUME_FLUX Real Normal velocity x vent area m3/s 0.Z0 Real Atmospheric profile origin m 10.

185

15.25 TABL (Table Parameters)

Table 15.25: For more information see Section 10.

TABL (Table Parameters)ID Character IDentifierFYI Character Comment String (has no effect)TABLE_DATA Real Array Data for one row of the table

15.26 TIME (Time Parameters)

Table 15.26: For more information see Section 6.2.

TIME (Time Parameters)DT Real Initial time step sFYI Character Comment String (has no effect)LOCK_TIME_STEP Logical Do not allow time step changes .FALSE.

RESTRICT_TIME_STEP Logical Do not allow time step to exceed initial .TRUE.

SYNCHRONIZE Logical Sync time step of multiple meshes .TRUE.

T_BEGIN Real Starting time for calculation s 0.T_END (TWFIN) Real Ending time for calculation s 1TIME_SHRINK_FACTOR Real See Section 6.2.3 1.WALL_INCREMENT Integer Time steps between 1D wall solution updates 2

15.27 TRNX, TRNY, TRNZ (MESH Transformations)

Table 15.27: For more information see Section 6.3.5.

TRNX, TRNY, TRNZ (MESH Transformations)CC Real Computational coordinate mFYI Character Comment String (has no effect)IDERIV Integer Order of polynomial transformationMESH_NUMBER Integer Number of mesh to transformPC Real Physical coordinate or derivative

15.28 VENT (Vent Parameters)

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Table 15.28: For more information see Section 7.4.

VENT (Vent Parameters)COLOR Character See Section 7.5CTRL_ID Character ID of Control FunctionDEVC_ID Character ID of Controlling DeviceDYNAMIC_PRESSURE Real See Section 9.3 Pa 0.0FYI Character Comment String (has no effect)IOR Integer Orientation IndexMASS_FRACTION(N) Real Array Mass Fraction of species N at OPEN vent kg/kgMB Character Mesh BoundaryOUTLINE Logical Draw vent as outline .FALSE.

PBX, PBY, PBZ Real Coordinate PlanePRESSURE_RAMP Character See Section 9.3RGB(3) Integer Triplet See Section 7.5SPREAD_RATE Real See Section 8.4.2 m/s 0.0SURF_ID Character Associated Surface ’INERT’

TEXTURE_ORIGIN(3) Real Triplet See Section 7.5.1 m (0.,0.,0.)TMP_EXTERIOR Real Temperature at OPEN vent CTRANSPARENCY Real Transparency indicator 1.0XB(6) Real Sextuplet Min/Max physical coordinates mXYZ(3) Real Triplet See Section 8.4.2 m

15.29 ZONE (Pressure Zone Parameters)

Table 15.29: For more information see Section 9.6.

ZONE (Pressure Zone Parameters)ID Character IDentifierLEAK_AREA(N) Real Leakage area to pressure zone N m2 0XB(6) Real Sextuplet Coordinates of Zone m

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Chapter 16

Conversion of Old Input Files to FDS 5

Many changes and improvements have been made in the latest release FDS 5. To make an FDS 4 input datafile compatible with the new FDS 5 application, a few changes must be made to the file. This appendix willpoint out all the changes that need to be made to convert an FDS 4.x input file to the new FDS 5.x format.

16.1 Numerical Domain Parameters: GRID and PDIM

In previous versions, the computational domain and numerical mesh were specified via lines of the form:

&GRID IBAR=30, JBAR=20, KBAR=10 /&PDIM XBAR0=0.0, XBAR=3.0, YBAR0=0.0, YBAR=2.0, ZBAR0=0.0, ZBAR=1.0 /

In FDS 5, these two lines are now written via the single line:

&MESH IJK=30,20,10, XB=0.0,3.0,0.0,2.0,0.0,1.0 /

Rules for multiple meshes and mesh transformations still apply.

16.2 Obstructions, Vents, and Holes: OBST, VENT, and HOLE

The syntax for these lines is fairly similar to past versions, with the following exceptions:

• For a VENT that spans an entire mesh boundary, CB=’XBAR0’ is now MB=’XMIN’. The character string’XBAR’ is now ’XMAX’. The same applies for the y and z coordinate parameters.

• Control parameters like T_ACTIVATE, HEAT_REMOVE, etc., are now consolidated into DEVC_ID andCTRL_ID. In brief, any change to an obstruction, vent, or hole is tied to a specific device or controlfunction. See Sections 13.1 and 13.5 for details.

16.3 Surface Parameters: SURF

The most significant change to the input file format has been splitting of the SURF line. In past versions, theSURF namelist group contained all the information about a particular boundary type – its material properties,color, thickness, and so on. However, in FDS 5, solid boundaries can now consist of multiple layers ofmaterials, making the old SURF line too cumbersome to specify. Instead, there is a new namelist groupcalled MATL that just contains the properties of a given material. What used to be

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&SURF ID = 'BRICK WALL'RGB = 0.6,0.2,0.2KS = 0.69C_P = 0.84DENSITY = 1600.BACKING = 'EXPOSED'THICKNESS = 0.20 /

is now given by two input lines:

&MATL ID = 'BRICK'CONDUCTIVITY = 0.69SPECIFIC_HEAT = 0.84DENSITY = 1600. /

&SURF ID = 'BRICK WALL'MATL_ID = 'BRICK'RGB = 166,41,41BACKING = 'EXPOSED'THICKNESS = 0.20 /

The surface is still specified the same way as before, for example:

&OBST XB=0.1, 5.0, 1.0, 1.2, 0.0, 1.0, SURF_ID='BRICK WALL' /

Notice the change in the names of the thermal properties KS and C_P to CONDUCTIVITY and SPECIFIC_HEAT,respectively. Notice that the color RGB is now specified via integers between 0 and 255, instead of real num-bers between 0.0 and 1.0. Better yet, just use the COLOR Table 7.1.

16.4 Reaction Parameters: REAC

For most applications, the specification of the combustion reaction has become easier. In past versions, youneeded to specify the fuel, its molecular weight, soot and/or CO yields, and the ideal stoichiometry of thereaction:

&REAC ID = 'PROPANE'FYI = 'C_3 H_8'MW = 44.SOOT_YIELD = 0.01NU_O2 = 5.NU_CO2 = 3.NU_H2O = 4. /

Now, you just need to describe the composition of the fuel molecule and any non-ideal product yield. FDS5 computes what it needs based on this information.

&REAC ID = 'PROPANE'SOOT_YIELD = 0.01C = 3.H = 8. /

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16.5 Device Parameters: SPRK, SMOD, HEAT, THCP

Past versions of FDS had a variety of ways to specify devices. For example, a sprinkler was specified via aline of the form:

&SPRK XYZ=4.5,6.7,3.6, MAKE='Acme_K-17', LABEL='spk_34' /

which located the sprinkler at XYZ and indicated that the sprinkler’s properties were listed in a file calledAcme_K-17.spk. Smoke and heat detectors were specified via lines of the form:

&SMOD XYZ=4.5,6.7,3.6, LENGTH=2.6, ACTIVATION_OBSCURATION=1.4, LABEL='sd_34' /&HEAT XYZ=4.5,6.7,3.6, RTI=45., ACTIVATION_TEMPERATURE=74., LABEL='hd_39' /

In FDS 5, these devices are all specified in the same way:

&PROP ID='Acme_K-17', QUANTITY='SPRINKLER LINK TEMPERATURE', RTI=148., C_FACTOR=0.7,ACTIVATION_TEMPERATURE=74., PART_ID='water drops', FLOW_RATE=189.3,DROPLET_VELOCITY=10., SPRAY_ANGLE=30.,80. /

&DEVC ID='spk_34', XYZ=4.5,6.7,3.6, PROP_ID='Acme_K-17' /

Point output via “thermocouples” (THCPs) are now given by “devices” (DEVCs):

&DEVC XYZ=0.7,0.9,2.1, QUANTITY='WALL TEMPERATURE', IOR=-2, ID='probe_2' /

The syntax of the old THCP namelist group is almost the same. Just swap DEVC for THCP, and change LABELto ID. In FDS 5, any input record is identified via its ID.

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Part III

FDS and Smokeview Development Tools

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Chapter 17

The FDS/Smokeview Repository

For those interested in obtaining the FDS and Smokeview source codes, either for development work orsimply to compile on a particular platform, it is strongly suggested that you download onto your computerthe entire FDS/Smokeview “Repository.” All project documents are maintained using the online utilityGoogle Code Project Hosting, a free service offered by Google to support software development for opensource applications. Google Code uses the Subversion (SVN) revision management system. Under thissystem a centralized repository containing all project files resides on a Google Code server. Subversion usesa single integer that identifies the version of the entire repository rather than of a specific file (i.e. anytime achange is made to the repository all files are incremented in version number). A record of version numberwhen a specific file was last changed is maintained.

As an open source program, any individual can obtain a copy of the repository or retrieve specificversions repository. Only the FDS and Smokeview developers can commit changes to the repository.

The current location of the FDS repository is http://fds-smv.googlecode.com/svn/trunk/.The repository contains the following files:

1. FDS and Smokeview source code files

2. FDS and Smokeview documentation

3. Input files for software testing (Examples), verification testing, and validation testing

4. Experimental data files used for validation testing

5. Scripts and post-processing utilities used for software testing

6. Web pages and wikis

The wikis are particularly useful in describing the details of how you go about working with the Repositoryassets.

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Chapter 18

Compiling FDS

This section describes what you need to know if you want to compile the FDS source code yourself. It isnot a step by step guide, more detailed instructions can be found on Developer section of the web site athttp://fire.nist.gov/fds.

If a compiled version of FDS exists for the machine on which the calculation is to be run and no changeshave been made to the original source code, there is no need to re-compile the code. For example, the filefds5.exe is the compiled single processor program for a Windows-based PC; thus PC users do not need aFortran compiler and do not need to compile the source code. For machines for which an executable has notbeen compiled, you must compile the code. Fortran 90/95 and C compilers are needed for compilation.

18.1 FDS Source Code

Table 18.1 lists the files that make up the source code. The files with suffix “.f90” contain free form Fortran90 instructions conforming to the ANSI and ISO standards, with a few exceptions that are discussed below.The source files should be compiled in the order in which they are listed in Table 18.1 because some routinesare dependent on others. For Unix/Linux users, Makefiles for various platforms are available that assist inthe compilation. Compiler options differ from platform to platform. Note the following:

• The source code consists mainly of Fortran 90 statements organized into about 25 files, plus an extra filecontaining some additional C routines needed for output to Smokeview. All of the C code is containedwithin the file called isob.c.

• Be aware that different compilers handle the names of C subroutines differently. Some compilers appendan underscore to the names of the C routines called by the Fortran code. If the compiler produces anerror involving the names of routines that are not recognized, invoke the C compiler pre-processingdirective pp_noappend to stop the compiler from appending the underscore to the names of the Croutines.

• There is only one non-standard call in the Fortran code. The non-standard call is GETARG, in func.f90.This routine reads the name of the input file off of the command line. This call cannot be simplycommented out; a suitable alternative must be found. The only compiler option necessary, in additionto any needed to address the above issues, is for full optimization (usually -O or some variant). Somecompilers have a standard optimization level, plus various degrees of “aggressive” optimization. Becautious in using the highest levels of optimization.

• For the single processor version of FDS, compile with mpis.f90

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• The parallel version of FDS uses mpip.f90 instead of mpis.f90, plus additional MPI libraries needto be installed. More details on MPI can be found at the web site, along with links to the necessaryorganizations who have developed free MPI libraries.

Table 18.1: Source Code Files

File Name Descriptionisob.c C Routine for computing isosurfaces and 3D smokeprec.f90 Specification of numerical precisionsmvv.f90 Interfaces for C routines used for Smokeview outputdevc.f90 Derived type definitions and constants for devices’type.f90 Derived type definitionsmesh.f90 Arrays and constants associated with each meshcons.f90 Global arrays and constantsfunc.f90 Global functions and subroutinesirad.f90 Functions needed for radiation solver, including RadCalieva.f90 Support routines for evac.f90evac.f90 Egress computations (future capability)pois.f90 Poisson (pressure) solverradi.f90 Radiation solverpart.f90 Lagrangian particle transport and sprinkler activationctrl.f90 Definitions and routines for control functionsdump.f90 Output data dumps into filesread.f90 Read input parametersmass.f90 Mass equation(s) and thermal boundary conditionswall.f90 Wall boundary conditionsfire.f90 Combustion routinespres.f90 Spatial discretization of pressure (Poisson) equationdivg.f90 Compute the flow divergenceinit.f90 Initialize variables and Poisson solverturb.f90 Experimental routines, mostly involving the turbulence modelvelo.f90 Momentum equationsvege.f90 Experimental vegetation modelmpis.f90 "Dummy" Fortran/MPI bindings for non-MPI compilationmpip.f90 MPI "include" statement for MPI compilationmain.f90 Main program for both serial and parallel versions

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Chapter 19

Output File Formats

The output from the code consists of the file CHID.out, plus various data files that are described below.Most of these output files are written out by the routine dump.f, and can easily be modified to accommodatevarious plotting packages.

19.1 Diagnostic Output

The file CHID.out consists of a list of the input parameters, and an accounting of various important quanti-ties, including CPU usage. Typically, diagnostic information is printed out every 100 time steps

.

.

Iteration 8300 May 16, 2003 08:37:53----------------------------------------------Mesh 1, Cycle 3427CPU/step: 2.272 s, Total CPU: 2.15 hrTime step: 0.03373 s, Total time: 128.86 sMax CFL number: 0.86E+00 at ( 21, 9, 80)Max divergence: 0.24E+01 at ( 25, 30, 22)Min divergence: -.39E+01 at ( 26, 18, 31)Number of Sprinkler Droplets: 615Total Heat Release Rate: 7560.777 kWRadiation Loss to Boundaries: 6776.244 kWMesh 2, Cycle 2914CPU/step: 1.887 s, Total CPU: 1.53 hrTime step: 0.03045 s, Total time: 128.87 sMax CFL number: 0.96E+00 at ( 21, 29, 42)Max divergence: 0.20E+01 at ( 22, 20, 22)Min divergence: -.60E+01 at ( 7, 26, 48)Number of Sprinkler Droplets: 301

.

.

The Iteration number indicates how many time steps the code has run, whereas the Cycle number for a givenmesh indicates how many time steps have been taken on that mesh. The date and time (wall clock time) areon the line starting with the word Iteration. The quantity CPU/step is the amount of CPU time requiredto complete a time step for that mesh; Total CPU is the amount of CPU time elapsed since the start ofthe run; Time step is the time step size for the given mesh; Total time is the time of the simulation;

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Max/Min divergence is the max/min value of the function ∇ ·u and is used as a diagnostic when the flowis incompressible (i.e. no heating); and Max CFL number is the maximum value of the CFL number. TheRadiation Loss to Boundaries is the amount of energy that is being radiated to the boundaries. Ascompartments heat up, the energy lost to the boundaries can grow to be an appreciable fraction of the TotalHeat Release Rate. Finally, Number of Tracer Particles indicates how many passive particlesare being tracked at that time.

Following the completion of a successful run, a summary of the CPU usage per subroutine is listed.This is useful in determining where most of the computational effort is being placed.

19.2 Heat Release Rate and Related Quantities

The heat release rate of the fire, plus other global energy-related quantities, are automatically written into atext file called CHID_hrr.csv. The format of the file is as follows

s,kW,kW,kW,kW,kg/s,atm,atmFDS_HRR_Time,HRR,RAD_LOSS,CONV_LOSS,COND_LOSS,BURN_RATE,ZONE_01,ZONE_02, ...0.0000000E+000, 0.0000000E+000, ...3.5355338E-001, 0.0000000E+000, ......

HRR is the total heat release rate, RAD_LOSS is the amount of thermal radiation lost to the boundaries,CONV_LOSS is the rate of energy that is flowing out of (positive) or into (negative) the computational domain,COND_LOSS is the rate of energy that is being conducted into (positive) or out of (negative) the solid surfaces,BURN_RATE is the total mass loss rate of fuel, and ZONE_01, etc., are the background pressures of the variouspressure ZONEs. Note that the reported BURN_RATE is not adjusted to account for the possibility that eachindividual material might have a different heat of combustion.

Details of the integrated energy quantities can be found in Section 14.3.1. The background pressure isdiscussed in Section 9.6.

19.3 Device Output Data

Data associated with particular devices (link temperatures, smoke obscuration, thermocouples, etc.) spec-ified in the input file under the namelist group DEVC is output in comma delimited format in a file calledCHID_devc.csv. The format of the file is as follows

s , UNITS(1) , UNITS(2) , ... , UNITS(N_DEVC)FDS Time , ID(1) , ID(2) , ... , ID(N_DEVC)T(1) , VAL(1,1) , VAL(2,1) , ... , VAL(N_DEVC,1)T(2) , VAL(1,2) , VAL(2,2) , ... , VAL(N_DEVC,2)

.

.

.

where N_DEVC is the number of devices, ID(I) is the user-defined ID of the Ith device, UNITS(I) theunits, T(J) the time of the Jth dump, and VAL(I,J) the value at the Ith device at the Jth time. The filescan be imported into Microsoft Excel or almost any other spread sheet program. If the number of columnsexceeds 256, the file will automatically be split into smaller files.

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19.4 Control Output Data

Data associated with particular control functions specified in the input file under the namelist group CTRL isoutput in comma delimited format in a file called CHID_ctrl.csv. The format of the file is as follows

s,status,status,status,statusFDS Time,ID(1),ID(2),...,ID(N_CTRL)0.00000E+000,-001, 001, ...1.11803E-001,-001,-001, ...

.

.

.

where N_CTRL is the number of controllers, ID(I) is the user-defined ID of the Ith control function, andplus or minus 1’s represent the state -1 = .FALSE. and +1 = .TRUE. of the Ith control function at theparticular time. The files can be imported into Microsoft Excel or almost any other spread sheet program. Ifthe number of columns exceeds 256, the file will automatically be split into smaller files.

19.5 Gas Mass Data

The total mass of the various gas species at any instant in time is reported in the comma delimited fileCHID_mass.csv. The file consists of several columns, the first column containing the time in seconds, thesecond contains the total mass of all the gas species in the computational domain in units of kg, the nextlines contain the total mass of the individual species.

You must specifically ask that this file be generated, as it can potentially cost a fair amount of CPU timeto generate. Set MASS_FILE=.TRUE. on the DUMP line to create this output file.

19.6 Mixture Fraction State Relations

The functional dependence of the mass fraction of the reactants and products of combustion on the mixturefraction is reported in the comma delimited file CHID_state.csv. The file consists of nominally 10 columns,the first column containing the mixture fraction, the last column the average molecular weight, and the restthe mass fractions of the various gas species in the case where complete combustion has occurred.

You must specifically ask that this file be generated. Set STATE_FILE=.TRUE. on the DUMP line tocreate this output file. If you forget to do this, you can easily re-run the case (remember to rename itsomething else!) for a few time steps. This file is created before the time iterations even begin.

19.7 Slice Files

The slice files defined under the namelist group SLCF are named CHID_n.sf (n=01,02...), and are writtenout unformatted, unless otherwise directed. These files are written out from dump.f with the followinglines:

WRITE(LUSF) QUANTITYWRITE(LUSF) SHORT_NAMEWRITE(LUSF) UNITSWRITE(LUSF) I1,I2,J1,J2,K1,K2WRITE(LUSF) TIMEWRITE(LUSF) (((QQ(I,J,K),I=I1,I2),J=J1,J2),K=K1,K2)

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.

.

.WRITE(LUSF) TIMEWRITE(LUSF) (((QQ(I,J,K),I=I1,I2),J=J1,J2),K=K1,K2)

QUANTITY, SHORT_NAME and UNITS are character strings of length 30. The sextuplet (I1,I2,J1,J2,K1,K2)denotes the bounding mesh cell nodes. The sextuplet indices correspond to mesh cell nodes, or corners, thusthe entire mesh would be represented by the sextuplet (0,IBAR,0,JBAR,0,KBAR).

There is a short Fortran 90 program provided, called fds2ascii.f, that can convert slice files into text filesthat can be read into a variety of graphics packages. The program combines multiple slice files correspond-ing to the same “slice” of the computational domain, time-averages the data, and writes the values into onefile, consisting of a line of numbers for each node. Each line contains the physical coordinates of the node,and the time-averaged quantities corresponding to that node. In particular, the graphics package Tecplotreads this file and produces contour, streamline and/or vector plots. See Section 14.4 for more details aboutthe program fds2ascii.

19.8 Plot3D Data

Quantities over the entire mesh can be output in a format used by the graphics package Plot3D. The Plot3Ddata sets are single precision (32 bit reals), whole and unformatted. Note that there is blanking, that is,blocked out data points are not plotted. If the statement WRITE_XYZ=.TRUE. is included on the DUMP line,then the mesh data is written out to a file called CHID.xyz

WRITE(LU13) IBAR+1,JBAR+1,KBAR+1WRITE(LU13) (((X(I),I=0,IBAR),J=0,JBAR),K=0,KBAR),

. (((Y(J),I=0,IBAR),J=0,JBAR),K=0,KBAR),

. (((Z(K),I=0,IBAR),J=0,JBAR),K=0,KBAR),

. (((IBLK(I,J,K),I=0,IBAR),J=0,JBAR),K=0,KBAR)

where X, Y and Z are the coordinates of the cell corners, and IBLK is an indicator of whether or not the cellis blocked. If the point (X,Y,Z) is completely embedded within a solid region, then IBLK is 0. Otherwise,IBLK is 1. Normally, the mesh file is not dumped.

The flow variables are written to a file called CHID_****_**.q, where the stars indicate a time at whichthe data is output. The file is written with the lines

WRITE(LU14) IBAR+1,JBAR+1,KBAR+1WRITE(LU14) ZERO,ZERO,ZERO,ZEROWRITE(LU14) ((((QQ(I,J,K,N),I=0,IBAR),J=0,JBAR),K=0,KBAR),N=1,5)

The five channels N=1,5 are by default the temperature (C), the u, v and w components of the velocity(m/s), and the heat release rate per unit volume (kW/m3). Alternate variables can be specified with the inputparameter PLOT3D_QUANTITY(1:5) on the DUMP line. Note that the data is interpolated at cell corners,thus the dimensions of the Plot3D data sets are one larger than the dimensions of the computational mesh.

Smokeview can display the Plot3D data. In addition, the Plot3D data sets can be read into some othergraphics programs that accept the data format. This particular format is very convenient, and recognized bya number of graphics packages, including AVS, IRIS Explorer and Tecplot 1.

1With the exception of Smokeview, the graphics packages referred to in this document are not included with the source code,but are commercially available.

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19.9 Boundary Files

The boundary files defined under the namelist group BNDF are named CHID_n.bf (n=0001,0002...), and arewritten out unformatted. These files are written out from dump.f with the following lines:

WRITE(LUBF) QUANTITYWRITE(LUBF) SHORT_NAMEWRITE(LUBF) UNITSWRITE(LUBF) NPATCHWRITE(LUBF) I1,I2,J1,J2,K1,K2,IOR,NB,NMWRITE(LUBF) I1,I2,J1,J2,K1,K2,IOR,NB,NM

.

.

.WRITE(LUBF) TIMEWRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2)WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2)

.

.

.WRITE(LUBF) TIMEWRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2)WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2)

.

.

.

QUANTITY, SHORT_NAME and UNITS are character strings of lengths 60, 30 and 30, respectively. NPATCH isthe number of planes (or “patches”) that make up the solid boundaries plus the external walls. The sextuplet(I1,I2,J1,J2,K1,K2) defines the cell nodes of each patch. IOR is an integer indicating the orientationof the patch (±1,±2,±3). You do not prescribe these. NB is the number of the boundary (zero for externalwalls) and NM is the number of the mesh. Note that the data is planar, thus one pair of cell nodes is the same.Presently, Smokeview is the only program available to view the boundary files.

19.10 Particle Data

Coordinates and specified quantities related to tracer particles, sprinkler droplets, and other Lagrangianparticles are written to a FORTRAN unformatted (binary) file called CHID.prt5. Note that the formatof this file has changed from previous versions (4 and below). The file consists of some header material,followed by particle data output every DT_PART seconds. The time increment DT_PART is specified on theDUMP line. It is T_END/NFRAMES by default. The header materials is written by the following FORTRANcode in the file called dump.f90.

WRITE(LUPF) ONE_INTEGER ! Integer 1 to check Endian-nessWRITE(LUPF) NINT(VERSION*100.) ! FDS version numberWRITE(LUPF) N_PART ! Number of PARTicle classesDO N=1,N_PART

PC => PARTICLE_CLASS(N)WRITE(LUPF) PC%N_QUANTITIES,ZERO_INTEGER ! ZERO_INTEGER is a place holderDO NN=1,PC%N_QUANTITIES

WRITE(LUPF) CDATA(PC%QUANTITIES_INDEX(NN)) ! 30 character output quantityWRITE(LUPF) UDATA(PC%QUANTITIES_INDEX(NN)) ! 30 character output units

ENDDOENDDO

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Note that the initial printout of the number 1 is used by Smokeview to determine the Endian-ness of the file.The Endian-ness has to do with the particular way real numbers are written into a binary file. The versionnumber is used to distinguish new versus old file formats. The parameter N_PART is not the number ofparticles, but rather the number of particle classes corresponding to the PART namelist groups in the inputfile. Every DT_PART seconds the coordinates of the particles and droplets are output as 4 byte reals:

WRITE(LUPF) REAL(T,FB) ! Write out the time T as a 4 byte realDO N=1,N_PART

WRITE(LUPF) NPLIM ! Number of particles in the PART classWRITE(LUPF) (XP(I),I=1,NPLIM),(YP(I),I=1,NPLIM),(ZP(I),I=1,NPLIM)WRITE(LUPF) (TA(I),I=1,NPLIM) ! Integer "tag" for each particleIF (PC%N_QUANTITIES > 0) WRITE(LUPF) ((QP(I,NN),I=1,NPLIM),NN=1,PC%N_QUANTITIES)

ENDDO

The particle “tag” is used by Smokeview to keep track of individual particles and droplets for the purposeof drawing streamlines. It is also useful when parsing the file. The quantity data, QP(I,NN), is used bySmokeview to color the particles and droplets. Note that it is now possible with the new format to color theparticles and droplets with several different quantities.

19.11 Profile Files

The profile files defined under the namelist group PROF are named CHID_prof_nn.csv (nn=01,02...), andare written out formatted. These files are written out from dump.f with the following line:

WRITE(LU_PROF) T,NWP+1,(X_S(I),I=0,NWP),(Q(I),I=0,NWP)

After the time T, the number of node points is given and then the node coordinates. These are written out atevery time step because the wall thickness and the local solid phase mesh may change over time due to thesolid phase reactions. Array Q contains the values of the output quantity, which may be wall temperature,density or component density.

19.12 3-D Smoke File Format

3-D smoke files contain alpha values used by Smokeview to draw semi-transparent planes representingsmoke and fire. FDS outputs 3-D smoke data at fixed time intervals, but unlike other output data, the fileformat is C, not Fortran. Note that char’s are one byte, and “int’s” and float’s are four bytes. A pseudo-coderepresentation of the 3-D smoke file is given by:

endian flag (int)is1, is2, js1, js2, ks1, ks2 (6*int)version (int)for each time:

time (float)chars_uncompressed, chars_compressed (2*int)compressed_data (chars_compressed*char)

end time

The endian flag is an integer one. Smokeview uses this number to determine whether the computer creatingthe 3-D smoke file and the computer viewing the 3-D smoke file use the same or different byte swap (endian)conventions for storing floating point numbers. The opacity data is compressed using run-length encoding(RLE).

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19.13 Isosurface File Format

Iso-surface files are used to store one or more surfaces where the specified QUANTITY is a specified value.FDS outputs iso-surface data at fixed time intervals. As with 3-D smoke, the file format is C, not Fortran.Note that char’s are one byte, short’s are two bytes and “int’s” and float’s are four bytes. A pseudo-coderepresentation of the iso-surface file is given by:

version (int)len1,len2,len3 (3*int)label1,label2,label3 ((len1+len2+len3+4)*char)nlevels (int)level_1, level_2, ..., level_nlevels (nlevels*float)for each time:

time (float)for each levelnvertices (int)ntriangles (int)vertices_1, ..., vertex_nvertices (3*short*nvertices)triangles_1, ..., triangle_ntriangles (3*(byte/short/float)*ntriangles)

end levelend time

The length of each triangles_i node is one byte if the number of triangles, ntriangles, is betweenzero and 255 (inclusive), two bytes if ntriangles is between 256 and 65536 (inclusive) and four bytes ifntriangles is greater than or equal to 65536.

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